Rear end gearing

[Sideflop]

In my F250's current state, it has axle code 73 (pretty sure that's a Dana 60 rear end with 3.54s), and the truck gets about 15-18 mpg going 65-75 mph. I'm sure that I'll be using it to haul or tow stuff occasionally. Would it be good to keep 3.54s, or is there a better rear end I should know about?

 

[Gary Lewis]

Yes, the 73 does indicate 3.54 gears. That's what Big Blue has and he's spinning ~2500 RPM at 65 MPH, which isn't conducive to decent MPG. But, he'll tow anything quite happily in 4th gear at that speed.

So I think the issue is not in the axle ratio but in not having an overdrive gear. We both need the engine turning in that range when working hard, but it sure would be nice to get better MPG - although your 15 - 18 MPG is already pretty decent when compared to my 11 - 12 MPG. Anyway, that would take dropping the RPM when cruising w/o a load. And that takes a tranny with OD.

Toward that end I'm installing a ZF5 behind Big Blue's 460. That'll give me the 2500 RPM at 65 MPH in 4th for towing, but drop it to 1900 in 5th for cruising. And then I might get 14 MPH.

[Sideflop]

Cool, thanks for the info.

[Steve83]

Within a fairly wide range, RPM is NOT directly related to MPG. Making the engine spin slower will make it work harder (higher compression/more throttle/more fuel per combustion event) to push the same truck over the same mile.

15-18MPG is pretty good for any F250; especially a 35-year-old diesel 4WD. If you want to increase it slightly, air up the tires, remove all unneeded weight, and drive like there's a tall wine glass on the dash - don't spill it (no hard acceleration, braking, or steering).

[1986F150Six]

Within a fairly wide range, RPM is NOT directly related to MPG. Making the engine spin slower will make it work harder (higher compression/more throttle/more fuel per combustion event) to push the same truck over the same mile.

15-18MPG is pretty good for any F250; especially a 35-year-old diesel 4WD. If you want to increase it slightly, air up the tires, remove all unneeded weight, and drive like there's a tall wine glass on the dash - don't spill it (no hard acceleration, braking, or steering).

Please understand that I am not being argumentative, as that is not my nature, but I do think engine speed can and does make a difference in fuel mileage.

With over 10 years of records as my truck is driven the same distance and route to and from work, the normal gas consumption is ~17+ mpg. Approximately 50% of this is @ 45 mph [the rest is stop and go]. At 45 mph, the truck can be in OD @ ~1200 RPMs.

Experimentation has shown that if OD is not used for the entire tank, the mileage drops to 15+ mpg. While in 3rd gear @ 45 mph, the RPMs are ~1675. Looking at the vacuum gauge reveals that the vacuum is greater when cruising in 3rd vs. OD.

Pumping losses take an effect.

 

[Gary Lewis]

Please understand that I am not being argumentative, as that is not my nature, but I do think engine speed can and does make a difference in fuel mileage.

With over 10 years of records as my truck is driven the same distance and route to and from work, the normal gas consumption is ~17+ mpg. Approximately 50% of this is @ 45 mph [the rest is stop and go]. At 45 mph, the truck can be in OD @ ~1200 RPMs.

Experimentation has shown that if OD is not used for the entire tank, the mileage drops to 15+ mpg. While in 3rd gear @ 45 mph, the RPMs are ~1675. Looking at the vacuum gauge reveals that the vacuum is greater when cruising in 3rd vs. OD.

Pumping losses take an effect.

David - Agreed.

The 80's were a 'tween time for trucks. The lighter trucks, like the F100's and some of the F150's, got really high gearing in a quest for MPG. For instance, the 1983 Fuel Saver Package got 2.47:1 gears, as shown on Page 28 of the 1983 Dealer Facts Book on the Vehicles and then 4x2 tabs. However, as David has explained before, while those gears gave good MPG, they weren't fun to drive as you had to downshift if there was a hill in sight.

But, since the HD trucks were meant for towing and hauling, they needed more reasonable gearing, like 3.54's. Unfortunately Ford didn't have transmissions with overdrive available for the big engines in the early part of the 80's. In fact, it wasn't until 1987 when the ZF5 came out that the 460 and diesel trucks had an OD manual tranny available, and it wasn't until '89 that there was an automatic with OD available for those engines.

Ford wouldn't have gone to the expense of making transmissions with overdrive gears available if they didn't help on the MPG. And they help because of two things. One, the higher the engine RPM the more internal friction there is. Second, as David said, the higher the vacuum the higher the pumping losses. Or, to say it another way, the lower the vacuum (due to gear selection) the lower the pumping losses are and, therefore, the higher the efficiency.

Here's a quote from Hard Working Trucks:

“Lower (higher numerically) axle ratios such as the 3.73s and 4.10s, provide the best acceleration and towing performance in pickups,” says Dawn Piechocki, Ford’s Vehicle Engineering Manager for the new F-150s, Expeditions and Navigators.

“But in real-world driving, unladen, those lower ratios cost about 2 mpg in highway fuel economy compared to 3.15s, which is the ratio [Ford] uses in pickups when trying to maximize mpg,” says Piechocki. “But in city driving the lower axle ratios have little effect of fuel mileage.”

And, in fact the 2 mpg difference is about the penalty I see on Blue by having spec'd the Max Tow package, which gave me 3.55 gears. I've checked with others that didn't tick that box and, therefore, got the taller 3.15 gears and they do get better MPG.

 

[1986F150Six]

And, in fact the 2 mpg difference is about the penalty I see on Blue by having spec'd the Max Tow package, which gave me 3.55 gears. I've checked with others that didn't tick that box and, therefore, got the taller 3.15 gears and they do get better MPG.

But, can those "others" effortlessly pull Big Blue on a trailer @ 75 mph?

 

[Gary Lewis]

And, in fact the 2 mpg difference is about the penalty I see on Blue by having spec'd the Max Tow package, which gave me 3.55 gears. I've checked with others that didn't tick that box and, therefore, got the taller 3.15 gears and they do get better MPG.

But, can those "others" effortlessly pull Big Blue on a trailer @ 75 mph?

LOL! Not as easily, that is for sure.

So, let me add to my statement. I'm glad that I picked the Max Tow package. I spec'd Blue to tow, and tow it will, with ease. As David said, my brother and I made a mad dash to Florida to get Big Blue, and towed him back through the night at 75 MPH. My brother was amazed at how easily it did it. No muss, no fuss.

Yes, I'd get better MPG with taller gearing. But I bought the truck to tow our boat and my car-hauler trailer, which is does superbly, and not to get the best MPG. To date I know of 5,000 miles of towing it has done in two trips - taking our 25' Sea Ray to Lake Powell and back, getting 11.0 MPG at a relaxed 65 MPH, and bringing Big Blue back and getting 9.0 MPG. Plus many shorter trips, like taking the trailer to Kansas to get the D60.

And our daughter is tentatively planning a road trip using it to tow a camping trailer of some kind next summer, and I know it'll do the job for them very nicely. So, I spec'd it correctly.

[85lebaront2]

LOL! Not as easily, that is for sure.

So, let me add to my statement. I'm glad that I picked the Max Tow package. I spec'd Blue to tow, and tow it will, with ease. As David said, my brother and I made a mad dash to Florida to get Big Blue, and towed him back through the night at 75 MPH. My brother was amazed at how easily it did it. No muss, no fuss.

Yes, I'd get better MPG with taller gearing. But I bought the truck to tow our boat and my car-hauler trailer, which is does superbly, and not to get the best MPG. To date I know of 5,000 miles of towing it has done in two trips - taking our 25' Sea Ray to Lake Powell and back, getting 11.0 MPG at a relaxed 65 MPH, and bringing Big Blue back and getting 9.0 MPG. Plus many shorter trips, like taking the trailer to Kansas to get the D60.

And our daughter is tentatively planning a road trip using it to tow a camping trailer of some kind next summer, and I know it'll do the job for them very nicely. So, I spec'd it correctly.

FWIW, very good friend has a 1995 F350 2WD crew cab dually with the Powerstroke 7.3L, E4OD and 4.10 gear, he gets 14 mpg normally and it doesn't seem to vary much with weight, air drag yes. Darth is a 1986 similar truck but 460 converted to EFI (but still has the deep dish pistons) and E4OD with a 3.55 gear. Right now I get an average of around 11 mpg, but have been as high as 12.5 on a nice high speed (70 mph) drive to W VA to my son's. New engine going in is 9.3:1 compression and mild cam.

I am hoping to get up around his fuel economy, as far as towing, I hauled a 10K 5th wheel trailer, original owner was a horse breeder and hauled some big trailers. Even with the very low compression, I can stomp the throttle from a standing start and spin all 4 rear wheels.

[Gary Lewis]

FWIW, very good friend has a 1995 F350 2WD crew cab dually with the Powerstroke 7.3L, E4OD and 4.10 gear, he gets 14 mpg normally and it doesn't seem to vary much with weight, air drag yes. Darth is a 1986 similar truck but 460 converted to EFI (but still has the deep dish pistons) and E4OD with a 3.55 gear. Right now I get an average of around 11 mpg, but have been as high as 12.5 on a nice high speed (70 mph) drive to W VA to my son's. New engine going in is 9.3:1 compression and mild cam.

I am hoping to get up around his fuel economy, as far as towing, I hauled a 10K 5th wheel trailer, original owner was a horse breeder and hauled some big trailers. Even with the very low compression, I can stomp the throttle from a standing start and spin all 4 rear wheels.

Big Blue currently gets 11+ on the highway, but he only has the T19 with 1:1 top gear, as opposed to the .71:1 of your E4OD. So where you are turning ~1800 RPM @ 65 MPH I'm turning 2500. And I'm hoping, like you, that with the ZF5, EFI, and a better engine Big Blue will get about 14 MPG. If so, we will have some of the most efficient 460's around.