F350 Gearing choices...

[Ford F834]

I know this would be a better question in the context of a build thread, but I am going to wing it out there anyway. I am trying to decide which drivetrain components to put in my F350. I have almost too many choices, and I could use some fresh input.

 

My axles are 4.10’s and I have at my disposal a 460 wide ratio ZF, a close ratio 7.3 ZF, a BW1345 transfer case with a GearVendors overdrive on it (rear axle overdrive only) and a BW1356 transfer case with some flanges to adapt a 2wd DNE2 overdrive to the rear output shaft. Below are some possibilities that I am considering:

 

1) Sell the overdrive units and re-gear the axles to 3.55’s. This would simplify the driveline and simplify operation by getting rid of the need for double overdrive. The down side is the final drive ratio (2.73) isn’t quite as tall as I want, and I would have to use the close ratio ZF for a decent start out gear. The close ratio also has a really high reverse, making back up driving obnoxious with 3.55’s unless shifted into low range on the transfer case.

2) Run the 460 ZF and GearVendors overdrive. This would be my easiest install. With 4.10’s 2nd gear starts are possible. I would have a crawl gear without shifting into low range. I would have a nice low geared reverse. The GVOD would produce a near ideal final drive ratio of 2.43 and works well as a splitter. The down side is the GVOD is a little clumsy to use with manual shift, only works in 2wd, and has a wearable conical clutch. The hydraulic pump and planetary design means that it cannot be engaged below 25 mph, and engine braking is particularly hard on the clutch (not good if you do a lot of towing in the mountains). Although Gear Vendors reps have told me it is plenty strong enough for my power level, it may not last as long as I’d like.

3) Run the 7.3 ZF and DNE2 overdrive. The reason I would pair the DNE2 with the close ratio ZF is because the DNE2 is a direct gear drive unit and can be left in either high or low range. If  left in high range, it would essentially act like a set of 3.28 axle gears. The cast iron gear case is physically about four times the size and weight of the GVOD and is rated for 35,000 lbs. combined vehicle weight. It is much more robust and has no wearable clutches. Unfortunately the electric motor shifts are slow, and not well suited to shifting in and out of overdrive as part of the normal driving sequence. (Same mechanism as an Eaton two speed axle). It has no lockout to prevent engagement in 4x4 which could result in severe vehicle damage if misused. Again the high reverse ratio would be a problem (see above).

4) Mount the DNE2 directly to the ZF 5 and source a divorced NP205. This last option is a variation on option 3. The DNE2’s input is identical to a 6 bolt transfer case and can therefore be mounted directly to a 4x4 transmission. The much more rare 4x4 version is actually designed to go in between the transmission and a married transfer case. Good luck finding one of those! But I can find divorced NP205’s for a price. This would make it useable in 4x4, and some great driveline parking brake set ups are available for the NP205. The down side is cost of the NP205, three new drive shafts, two more U joints and a lot of cross member fabrication. Bullet proof but no doubt more driveline friction loss than other options.

There isn’t a right or best answer, I’m just curious what you all would pick in my situation. Or maybe something that I have not suggested here. Also, both ZF’s should be rebuilt, and probably the BW1356 if I use it. I’m not particularly interested in going to a ZF-6 although it can be done, and I’m not wanting to convert to automatic.

[Gary Lewis]

Shades of an 8-page microanalysis!

No, this doesn't belong buried in a build thread. It needs to be a stand-alone topic, like this one. So, let's get started.....

First, I get lost in all the gearing issues, numbers, etc. So, if you'll provide me with the gear ratios I'd like to create a spreadsheet to help me/us get our heads around the possibilities. And, that will let me compare the 2nd gear ratios with those of Big Blue, which has a fairly easy start in 2nd when running light and the 460 may have comparable torque at really low R's to the diesel.

Second, I don't like any of the combinations that would let you destroy things by placing it in 4lo. I don't want to build things that are easily destroyed, and that looks like a prime candidate for destruction. So I guess that w/o info on the gearing I'm knocking on Door #4. NP205's are said to be STOUT, albeit heavy. So that seems like the best option to make everything live w/the power you are planning for that engine to produce. But, that begs the question of gearing, so we've come back around the bush.....

 

[85lebaront2]

Damn, interesting thoughts on all of them.

Gear Vendors unit sounds like the English Laycock de Normanville unit used on Jaguars, Austin-Heally 3000s and in a lighter version MGs. I believe either Chrysler or AMC used them in the 70s on their manual transmission cars as an option. I do know that once you have reached the needed speed they can be shifted in or out with a flip or slide of the switch. You are correct on the cone clutch being the weak link, but, there is also a sprag clutch that carries the output during the shift up or down and it is where I have personally seen failures on the MG units.

DNE2 Would it be possible to convert the shifting system from a worm drive electric motor to either a vacuum or air pressure system? This could also incorporated a 4X4 lockout similar to the late model shifter interlocks (depress brake to shift from Park)

Gearing I am running a 3.55 rear in Darth, plenty of power and pretty good acceleration even with a well used low compression (lower than normal due to early EFI heads on a carbureted short block). I can even, if I stomp the throttle from a standstill light all 4 rear tires.

[Gary Lewis]

Damn, interesting thoughts on all of them.

Gear Vendors unit sounds like the English Laycock de Normanville unit used on Jaguars, Austin-Heally 3000s and in a lighter version MGs. I believe either Chrysler or AMC used them in the 70s on their manual transmission cars as an option. I do know that once you have reached the needed speed they can be shifted in or out with a flip or slide of the switch. You are correct on the cone clutch being the weak link, but, there is also a sprag clutch that carries the output during the shift up or down and it is where I have personally seen failures on the MG units.

DNE2 Would it be possible to convert the shifting system from a worm drive electric motor to either a vacuum or air pressure system? This could also incorporated a 4X4 lockout similar to the late model shifter interlocks (depress brake to shift from Park)

Gearing I am running a 3.55 rear in Darth, plenty of power and pretty good acceleration even with a well used low compression (lower than normal due to early EFI heads on a carbureted short block). I can even, if I stomp the throttle from a standstill light all 4 rear tires.

I was wondering about a lockout as well, but I'm thinking electrical is easier. Just put a relay in the circuit from the t-case.

[Nothing Special]

I was wondering about a lockout as well, but I'm thinking electrical is easier. Just put a relay in the circuit from the t-case.

One of my many plans for my '97 F-250 (and maybe I'll implement a few of them!) is to add a Gear Vendors to go with the 460, E4OD and 4.10 gears. I forget the Gear Vendors ratio, but as I recall, it would give me about a 3.2 axle ratio (not considering the trans gearing). That seemed like a great cruising ratio while still letting me keep the GCWR of the 4.10 ratio. (I had a 351W/E4OD/3.55/33" setup in an F-150. That actually even towed pretty well, so I'm not concerned about 3.2 with a 460 for cruising).

The Gear Vendors does have an electrical lockout to keep you from engaging it while the transfer case is in 4WD. The thing I think you'd still need to be careful about is not shifting the transfer case into 4WD while the Gear Vendors is already engaged. I'd assume the Gear Vendors would kick out pretty quick, but I don't imagine the transfer case would appreciate being engaged while the front and rear driveshafts were turning different speeds (assuming the hubs were already locked). But I don't think that would be a huge issue for me. It would just mean not being able to completely shut off my brain while driving in snow (the only time I'm likely to shift into 4WD when I might be in overdrive). And staying alert while driving in snow is a pretty good idea anyway.

But I know nothing about the durability of the Gear Vendors. I know a lot of people like it, but that's as far as my "knowledge" goes. So I'm definitely interested in this thread.

[Ford F834]

Gary, I have over 20 spreadsheet pages dedicated to gear ratios, tire sizes, rpm at different speeds etc., I would say that I have probably put too much thought and emphasis on the ratios, and not enough on the practicality of the equipment which is why I only hinted at the ratios in my first post. I will go into the ratio choices a little more at this point, but all else being equal/similar I wanted some raw comments on the set ups themselves.

 

As you said, I am not thrilled with any of the choices. They all have some pretty hefty drawbacks. I should, however, clarify a couple of things. First, the GVOD clutch is engaged with an electric solenoid. The controller has safety lock outs to prevent engagement in 4x4, while in reverse, and below 25mph. That pretty much ensures that the really bad thing won’t happen on a 4x4 unless you bypass the controller or hotwire something. The clutch disengages instantly when power is cut to the solenoid. With the DNE2, it is electrically shifted but stays mechanically engaged. One could devise a lockout that prevents shifting while the 4x4 light is on, but once it is in overdrive there is no way to prevent moving the transfer case lever. If the hubs are locked it will be game over for the weakest link in the driveline. There is no way around it. If you set up a 2wd DNE2 on the back end of a transfer case you have to be careful. Since the hubs are manual, getting out of the truck is a good reminder not to engage it but it can still happen.

 

Both units are quite strong. I called and talked to the Gear Vendors rep several years ago and he told me the unit I have is rated for trucks up to 1,200 horsepower. He said the flywheel HP must be multiplied by the ratio of any given gear, but you can’t use it in granny low or your start out gear anyway because of the speed lockout. And even then, the horsepower rating is provided you actually gain traction at that level. He said it is almost impossible to break the GVOD. The only real issue is eventual clutch wear. This is amazing to me, since the GVOD is such a tiny little thing. It just does not look like a brute, but evidently it is tough. The DNE2 is a monster by comparison. It reminds me of maybe a half size T18. Justin Anderson has one in his moderately built IDI diesel, and has done sled pulling competitions with no ill to the overdrive. It can take whatever I might throw at it. His only complaint is that he has gone through a couple of the $125 shift motors. If you have an old style (and he does) the agar limit switch is janky and can let the motor over travel and burn up. Mine has the new solid state controller with a timer. You can also use a momentary toggle and hold it 1-2 seconds… just long enough to load the shift spring. Then at the next clutch release or easing off the throttle the shift will happen.

 

Another anecdote regarding the GVOD toughness is a story some of you may remember me telling. I was on my way over Tahachapi pass in California when my speedometer cable started to bind. This caused an intermittent signal from the speed sensor to the GVOD controller. The result was that it began slamming in and out of overdrive at 70mph. This was extremely violent. It felt like I was being rear ended by a semi over and over until I figured out what was going on and disengaged the pull switch. One of the so-called “benefits” of the GVOD is that it can be shifted under full power. I whole heartedly don’t recommend this… at least not with a standard transmission unless you like whiplash. This little event did not improve my opinion of the product. I already didn’t care for it much because of the rattling push pull switch and clunky shift into overdrive. In start/stop driving it was easy to forget to push the switch back in, so when you went to engage the overdrive it was still pulled out. You had to then push it in and pull it again by which time you lost momentum and would lug into overdrive gear. A ZF-5 would not be as bad since you would not need it until higher speed. With the T19 and 4.10’s I was in overdrive by 35-40mph, so that exercise got old driving around town. From what I read, the DNE2 would not be fun to shift in and out of as part of your shift sequence. It takes a lot of getting used to and the shifts are slow. It would pretty much have to be left in high range (all gears 1 through 5 reduced by .80) or shifted into low range for towing heavy or 4x4 use.

 

So, about those ratios. Let me say that my baseline has been years of driving ¾ ton Fords with T18 four speeds and 4.10 axle gears with 235/85R16 tires (~31” diameter). The T18 ratios are:

 

1st 6.23 x 4.10 = 25.91

2nd 3.09 x 4.10 = 12.67

3rd 1.69 x 4.10 = 6.93

4th 1.00 x 4.10 = 4.10

 

Skipping the straight cut granny gear basically gives you a 12-7-4 three speed that begs for at least one more highway gear. This is what prompted me to use the T19 diesel transmission in my ’81. I have 4 Lo for a crawl gear if needed. As you can see it gave me roughly 12-7-4-3, and is almost perfect. It could ~maybe use a little bit more overdrive on level ground, but in hilly mountain terrain and the power level of a stock straight six I’m pretty much there. For the diesel I want a taller final drive for best mpg and cabin noise.

 

1st 4.02 x 3.00 = 12.06

2nd 2.41 x 3.00 = 7.23

3rd 1.41 x 3.00 = 4.23

4th 1.00 x 3.00 = 3.00

 

The GVOD has a reduction ratio of .78 and the DNE2 has a reduction ratio of .80, so below are some of the proposed combinations:

 

Close ratio (diesel) ZF with 3.55’s no auxiliary overdrive (everything is a little lower than desired):

1st 4.14 x 3.55 = 14.70

2nd 2.37 x 3.55 = 8.41

3rd 1.42 x 3.55 = 5.04

4th 1.00 x 3.55 = 3.55

5th 0.77 x 3.55 = 2.73

 

Close ratio (diesel) ZF with 4.10’s and GVOD (can’t split until 3rd, split 3rd and 4th are very redundant very low start out gear):

1st 4.14 x 4.10 = 16.97

2nd 2.37 x 4.10 = 9.72

3rd 1.42 x 4.10 = 5.82

3rd OD 1.11 x 4.10 = 4.54

4th 1.00 x 4.10 = 4.10

4th OD 0.78 x 4.10 = 3.20

5th 0.77 x 4.10 = 3.16

5th OD 0.60 x 4.10 = 2.46

 

Wide ratio (460) ZF with 3.55’s no auxiliary overdrive probably requires granny start out:

1st 5.72 x 3.55 = 20.31

2nd 2.94 x 3.55 = 10.44

3rd 1.61 x 3.55 = 5.72

4th 1.00 x 3.55 = 3.55

5th 0.76 x 3.55 = 2.70

 

Wide ratio (460) ZF with 4.10’s and GVOD (split 3rd possibly useful)

1st 5.72 x 4.10 = 23.45

2nd 2.94 x 4.10 = 12.05

3rd 1.61 x 4.10 = 6.60

3rd OD 1.26 x 4.10 = 5.15

4th 1.00 x 4.10 = 4.10

4th OD 0.78 x 4.10 = 3.20

5th 0.76 x 4.10 = 3.12

5th OD 0.59 x 4.10 = 2.43

 

Close ratio (diesel) ZF with 4.10’s and DNE2 in HIGH range:

1st 3.31 x 4.10 = 13.58

2nd 1.90 x 4.10 = 7.77

3rd 1.14 x 4.10 = 4.66

4th 0.80 x 4.10 = 3.28

5th 0.62 x 4.10 = 2.53

 

When my truck had the close ratio T19 and 4.10’s my 1st gear was 16.48 and it drove me nuts for casual driving while empty. It was obnoxiously low. 2nd gear was 9.88 and was pretty much impossible to start in unless slightly rolling or on a dead flat and I used lots of clutch finessing. I think that may have been partly the clutch’s fault, but a 4x4 crew cab is a lot of weight to get rolling. I believe I really need a start out gear in the 12-13 range. For overdrive gearing, 2.50 is right about optimal Justin ran the close ratio ZF, 4.10’s and the DNE2… but he also has 35” tires. If you factor that in, his final drive ratio would have been 2.13 on stock wheels. He said that he could feel it bog at inclines that you could hardly perceive visually, or in a headwind. The bogging would also show on the pyrometer in the form of rising EGT’s. He did a comparison mpg calculation and there was no difference between straight 5th gear and 5th + OD. So I am going to say that he is slightly over geared at that level. When I drove Jan’s truck to California (close ratio ZF and 3.55’s) the 2.73 drive ratio returned excellent fuel mileage, but it felt like it could use a little bit more gear at 70-75 mph freeway speeds. So I am hoping to come in around the 2.50 mark +/- with stock tires. I have run calculations with different tire sizes, but I don’t particularly want to go with bigger tires. They are expensive. The stock ones are skinny and have good traction in snow. They are relatively affordable and easy to find. I want the truck to have a more or less stock height and stance.

 

Okay, this monster post is DONE for now… welcome to my overthinking nightmare!

[Gary Lewis]

Gary, I have over 20 spreadsheet pages dedicated to gear ratios, tire sizes, rpm at different speeds etc., I would say that I have probably put too much thought and emphasis on the ratios, and not enough on the practicality of the equipment which is why I only hinted at the ratios in my first post. I will go into the ratio choices a little more at this point, but all else being equal/similar I wanted some raw comments on the set ups themselves.

 

As you said, I am not thrilled with any of the choices. They all have some pretty hefty drawbacks. I should, however, clarify a couple of things. First, the GVOD clutch is engaged with an electric solenoid. The controller has safety lock outs to prevent engagement in 4x4, while in reverse, and below 25mph. That pretty much ensures that the really bad thing won’t happen on a 4x4 unless you bypass the controller or hotwire something. The clutch disengages instantly when power is cut to the solenoid. With the DNE2, it is electrically shifted but stays mechanically engaged. One could devise a lockout that prevents shifting while the 4x4 light is on, but once it is in overdrive there is no way to prevent moving the transfer case lever. If the hubs are locked it will be game over for the weakest link in the driveline. There is no way around it. If you set up a 2wd DNE2 on the back end of a transfer case you have to be careful. Since the hubs are manual, getting out of the truck is a good reminder not to engage it but it can still happen.

 

Both units are quite strong. I called and talked to the Gear Vendors rep several years ago and he told me the unit I have is rated for trucks up to 1,200 horsepower. He said the flywheel HP must be multiplied by the ratio of any given gear, but you can’t use it in granny low or your start out gear anyway because of the speed lockout. And even then, the horsepower rating is provided you actually gain traction at that level. He said it is almost impossible to break the GVOD. The only real issue is eventual clutch wear. This is amazing to me, since the GVOD is such a tiny little thing. It just does not look like a brute, but evidently it is tough. The DNE2 is a monster by comparison. It reminds me of maybe a half size T18. Justin Anderson has one in his moderately built IDI diesel, and has done sled pulling competitions with no ill to the overdrive. It can take whatever I might throw at it. His only complaint is that he has gone through a couple of the $125 shift motors. If you have an old style (and he does) the agar limit switch is janky and can let the motor over travel and burn up. Mine has the new solid state controller with a timer. You can also use a momentary toggle and hold it 1-2 seconds… just long enough to load the shift spring. Then at the next clutch release or easing off the throttle the shift will happen.

 

Another anecdote regarding the GVOD toughness is a story some of you may remember me telling. I was on my way over Tahachapi pass in California when my speedometer cable started to bind. This caused an intermittent signal from the speed sensor to the GVOD controller. The result was that it began slamming in and out of overdrive at 70mph. This was extremely violent. It felt like I was being rear ended by a semi over and over until I figured out what was going on and disengaged the pull switch. One of the so-called “benefits” of the GVOD is that it can be shifted under full power. I whole heartedly don’t recommend this… at least not with a standard transmission unless you like whiplash. This little event did not improve my opinion of the product. I already didn’t care for it much because of the rattling push pull switch and clunky shift into overdrive. In start/stop driving it was easy to forget to push the switch back in, so when you went to engage the overdrive it was still pulled out. You had to then push it in and pull it again by which time you lost momentum and would lug into overdrive gear. A ZF-5 would not be as bad since you would not need it until higher speed. With the T19 and 4.10’s I was in overdrive by 35-40mph, so that exercise got old driving around town. From what I read, the DNE2 would not be fun to shift in and out of as part of your shift sequence. It takes a lot of getting used to and the shifts are slow. It would pretty much have to be left in high range (all gears 1 through 5 reduced by .80) or shifted into low range for towing heavy or 4x4 use.

 

So, about those ratios. Let me say that my baseline has been years of driving ¾ ton Fords with T18 four speeds and 4.10 axle gears with 235/85R16 tires (~31” diameter). The T18 ratios are:

 

1st 6.23 x 4.10 = 25.91

2nd 3.09 x 4.10 = 12.67

3rd 1.69 x 4.10 = 6.93

4th 1.00 x 4.10 = 4.10

 

Skipping the straight cut granny gear basically gives you a 12-7-4 three speed that begs for at least one more highway gear. This is what prompted me to use the T19 diesel transmission in my ’81. I have 4 Lo for a crawl gear if needed. As you can see it gave me roughly 12-7-4-3, and is almost perfect. It could ~maybe use a little bit more overdrive on level ground, but in hilly mountain terrain and the power level of a stock straight six I’m pretty much there. For the diesel I want a taller final drive for best mpg and cabin noise.

 

1st 4.02 x 3.00 = 12.06

2nd 2.41 x 3.00 = 7.23

3rd 1.41 x 3.00 = 4.23

4th 1.00 x 3.00 = 3.00

 

The GVOD has a reduction ratio of .78 and the DNE2 has a reduction ratio of .80, so below are some of the proposed combinations:

 

Close ratio (diesel) ZF with 3.55’s no auxiliary overdrive (everything is a little lower than desired):

1st 4.14 x 3.55 = 14.70

2nd 2.37 x 3.55 = 8.41

3rd 1.42 x 3.55 = 5.04

4th 1.00 x 3.55 = 3.55

5th 0.77 x 3.55 = 2.73

 

Close ratio (diesel) ZF with 4.10’s and GVOD (can’t split until 3rd, split 3rd and 4th are very redundant very low start out gear):

1st 4.14 x 4.10 = 16.97

2nd 2.37 x 4.10 = 9.72

3rd 1.42 x 4.10 = 5.82

3rd OD 1.11 x 4.10 = 4.54

4th 1.00 x 4.10 = 4.10

4th OD 0.78 x 4.10 = 3.20

5th 0.77 x 4.10 = 3.16

5th OD 0.60 x 4.10 = 2.46

 

Wide ratio (460) ZF with 3.55’s no auxiliary overdrive probably requires granny start out:

1st 5.72 x 3.55 = 20.31

2nd 2.94 x 3.55 = 10.44

3rd 1.61 x 3.55 = 5.72

4th 1.00 x 3.55 = 3.55

5th 0.76 x 3.55 = 2.70

 

Wide ratio (460) ZF with 4.10’s and GVOD (split 3rd possibly useful)

1st 5.72 x 4.10 = 23.45

2nd 2.94 x 4.10 = 12.05

3rd 1.61 x 4.10 = 6.60

3rd OD 1.26 x 4.10 = 5.15

4th 1.00 x 4.10 = 4.10

4th OD 0.78 x 4.10 = 3.20

5th 0.76 x 4.10 = 3.12

5th OD 0.59 x 4.10 = 2.43

 

Close ratio (diesel) ZF with 4.10’s and DNE2 in HIGH range:

1st 3.31 x 4.10 = 13.58

2nd 1.90 x 4.10 = 7.77

3rd 1.14 x 4.10 = 4.66

4th 0.80 x 4.10 = 3.28

5th 0.62 x 4.10 = 2.53

 

When my truck had the close ratio T19 and 4.10’s my 1st gear was 16.48 and it drove me nuts for casual driving while empty. It was obnoxiously low. 2nd gear was 9.88 and was pretty much impossible to start in unless slightly rolling or on a dead flat and I used lots of clutch finessing. I think that may have been partly the clutch’s fault, but a 4x4 crew cab is a lot of weight to get rolling. I believe I really need a start out gear in the 12-13 range. For overdrive gearing, 2.50 is right about optimal Justin ran the close ratio ZF, 4.10’s and the DNE2… but he also has 35” tires. If you factor that in, his final drive ratio would have been 2.13 on stock wheels. He said that he could feel it bog at inclines that you could hardly perceive visually, or in a headwind. The bogging would also show on the pyrometer in the form of rising EGT’s. He did a comparison mpg calculation and there was no difference between straight 5th gear and 5th + OD. So I am going to say that he is slightly over geared at that level. When I drove Jan’s truck to California (close ratio ZF and 3.55’s) the 2.73 drive ratio returned excellent fuel mileage, but it felt like it could use a little bit more gear at 70-75 mph freeway speeds. So I am hoping to come in around the 2.50 mark +/- with stock tires. I have run calculations with different tire sizes, but I don’t particularly want to go with bigger tires. They are expensive. The stock ones are skinny and have good traction in snow. They are relatively affordable and easy to find. I want the truck to have a more or less stock height and stance.

 

Okay, this monster post is DONE for now… welcome to my overthinking nightmare!

Wow! Just WOW! You think like I do - and I'm not sure that's a compliment. I'm not even sure where to start.....

Ok, the last bit on the target gear ratios is really the key to me. But, I beg to differ. Big Blue's T-19 has the same 2nd gear (3.09) as your T-18, I think, and with 3.55's and big tires the 10.97 combined ratio is fine for starting out. It might be ever so slightly too high, but it is quite acceptable. And I don't think the ZF's 10.44 combo is going to be a problem since the warmed-over 351M pulled it fine in Dad's truck. Big Blue has much larger tires, so the effective ratio was lower in Dad's, but the torque was lower as well. And your diesel will surely have far more torque on that bottom end than even the 460. So I think a bare wide-ratio ZF5 would do a good job on the bottom end, even with 3.55 gears, and it would be even better with 4.10's.

Then there's the cruising/top gear, and I don't have a clue what R's a diesel wants to turn, so I'll accept your 2.50 goal. The wide-ratio ZF/GV combo comes in pretty close @ 2.43. But, the ZF by itself isn't too far off at 2.70. So, what is the RPM difference at 70 MPH between the two combinations? (I'd run the numbers myself but I don't know that you said what tires you are planning on using.) Is it enough to be worrying about?

I'm wondering if a wide-ratio ZF/3.55 setup wouldn't fit the bill pretty closely. I know that's not what you want to do, but humor me for a minute. Is there a way you can try that combo before moving to an overdrive setup? I doubt it as I think you are going to need to have driveshafts made for each combination, and that's not cheap.

Speaking of cheap, what gears are in the truck now? Changing those is a considerable expense, so maybe we should think through the costs of doing whatever you decide and see if there's an easier, cheaper solution?

Anyway, I'm just trying to come at it from another angle or perspective.

 

[Ford F834]

Wow! Just WOW! You think like I do - and I'm not sure that's a compliment. I'm not even sure where to start.....

Ok, the last bit on the target gear ratios is really the key to me. But, I beg to differ. Big Blue's T-19 has the same 2nd gear (3.09) as your T-18, I think, and with 3.55's and big tires the 10.97 combined ratio is fine for starting out. It might be ever so slightly too high, but it is quite acceptable. And I don't think the ZF's 10.44 combo is going to be a problem since the warmed-over 351M pulled it fine in Dad's truck. Big Blue has much larger tires, so the effective ratio was lower in Dad's, but the torque was lower as well. And your diesel will surely have far more torque on that bottom end than even the 460. So I think a bare wide-ratio ZF5 would do a good job on the bottom end, even with 3.55 gears, and it would be even better with 4.10's.

Then there's the cruising/top gear, and I don't have a clue what R's a diesel wants to turn, so I'll accept your 2.50 goal. The wide-ratio ZF/GV combo comes in pretty close @ 2.43. But, the ZF by itself isn't too far off at 2.70. So, what is the RPM difference at 70 MPH between the two combinations? (I'd run the numbers myself but I don't know that you said what tires you are planning on using.) Is it enough to be worrying about?

I'm wondering if a wide-ratio ZF/3.55 setup wouldn't fit the bill pretty closely. I know that's not what you want to do, but humor me for a minute. Is there a way you can try that combo before moving to an overdrive setup? I doubt it as I think you are going to need to have driveshafts made for each combination, and that's not cheap.

Speaking of cheap, what gears are in the truck now? Changing those is a considerable expense, so maybe we should think through the costs of doing whatever you decide and see if there's an easier, cheaper solution?

Anyway, I'm just trying to come at it from another angle or perspective.

Gary, the truck currently has 4.10 gears. So my proposal to switch to 3.55’s would entail selling off the GVOD and DNE2 to fund that venture. I realize that is probably a good $1,000 to $1,200 to have both axles done.

Perhaps my experience with 2nd gear starts had more to do with the quality and diameter of the clutch. When I removed it, it had plenty left on the friction disc and looked to be in good condition, but maybe it just wasn’t big enough for the weight of the truck? 🤷‍♂️ The ZF clutch I have to go in it is going to be a 13” Southbend SD clutch on a modified 6.9 flywheel.

As for the top end, the IDI has its torque peak at 1,400rpm. My engine was always at its very happiest between 1,500 and 2,000. For pulling, 2,000 was the sweet spot... right at 45mph with no overdrive 55mph with overdrive. I know, this was a stock compression NA 6.9, and I will be running a very modified turbocharged 7.3 with 18:1 CR. My rpm sweet spot may be higher. I can’t find my pic of the wide ratio rpm graph, but below is one for the close ratio ZF using 4.10’s and 3.55’s

Note that the 2,000 rpm sweet spot with 3.55’s in 5th is right around 67mph, with 4.10’s in 5th +OD it is 75mph on the nose.

[Ford F834]

Gary, the truck currently has 4.10 gears. So my proposal to switch to 3.55’s would entail selling off the GVOD and DNE2 to fund that venture. I realize that is probably a good $1,000 to $1,200 to have both axles done.

Perhaps my experience with 2nd gear starts had more to do with the quality and diameter of the clutch. When I removed it, it had plenty left on the friction disc and looked to be in good condition, but maybe it just wasn’t big enough for the weight of the truck? 🤷‍♂️ The ZF clutch I have to go in it is going to be a 13” Southbend SD clutch on a modified 6.9 flywheel.

As for the top end, the IDI has its torque peak at 1,400rpm. My engine was always at its very happiest between 1,500 and 2,000. For pulling, 2,000 was the sweet spot... right at 45mph with no overdrive 55mph with overdrive. I know, this was a stock compression NA 6.9, and I will be running a very modified turbocharged 7.3 with 18:1 CR. My rpm sweet spot may be higher. I can’t find my pic of the wide ratio rpm graph, but below is one for the close ratio ZF using 4.10’s and 3.55’s

Note that the 2,000 rpm sweet spot with 3.55’s in 5th is right around 67mph, with 4.10’s in 5th +OD it is 75mph on the nose.

I wish I could trial a 10-ish start out gear, but not only will the driveshaft be specific, I don’t have an easy way to simulate the gearing. I just know with the old set up 9.88 was absolutely not viable. I know ratio wise the wide ratio with 4.10’s + GVOD would nearly ideal. It would just be nice to ditch the extra gear box.

My expected power level will be around 400 hp and 800+ tq at the rear wheels. But that of course is after the turbo spools. Off the line my torque may be similar to NA, if not lower because of the 18:1 decompression. Such is the performance compromise. 😐

Edit: I know that ideally one does not pull ZF’s for fun, but hypothetically if I did regear to 3.55 I could install the wide ratio ZF without rebuilding it. If I like it, then drop and rebuild. If not, then rebuild the close ratio and swap them. It would entail some extra work, but worst case I would end up with the close ZF/3.55 combo

[grumpin]

I wish I could trial a 10-ish start out gear, but not only will the driveshaft be specific, I don’t have an easy way to simulate the gearing. I just know with the old set up 9.88 was absolutely not viable. I know ratio wise the wide ratio with 4.10’s + GVOD would nearly ideal. It would just be nice to ditch the extra gear box.

My expected power level will be around 400 hp and 800+ tq at the rear wheels. But that of course is after the turbo spools. Off the line my torque may be similar to NA, if not lower because of the 18:1 decompression. Such is the performance compromise. 😐

Edit: I know that ideally one does not pull ZF’s for fun, but hypothetically if I did regear to 3.55 I could install the wide ratio ZF without rebuilding it. If I like it, then drop and rebuild. If not, then rebuild the close ratio and swap them. It would entail some extra work, but worst case I would end up with the close ZF/3.55 combo

Wow! Interesting.

For me, simplicity is becoming the norm. I would go with number 1, but if financially possible I would not sell the O/D units. Never know, might need them, or change your mind.

As Lebaron said, I also have 3.55's, (always had trucks with 4.10's), the 3.55's are great, good acceleration (the 460 definitely helps!) and not too bad on the RPM's if you're not trying to keep up on a freeway. I have the C6. (So I'm the "old man" doing 60MPH!)

You can go into 4 Low for backing and other situations.

I appreciate your thoughts on the overdrives as I have thought it would be nice to have one. But well over 3 grand will buy a lot of gas!