Rear end gearing

[grumpin]

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.

That’s funny right there!

 

[Gary Lewis]

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.

That’s funny right there!

Yeah, it was meant that way. But in reality it is pathetic.

[Nothing Special]

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....

.... 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....

With a gas engine lower vacuum (more open throttle) gives more efficiency, partly because of the reduced pumping loses, the engine doesn't have to work to pull air past the closed throttle plate. Back in a college Internal Combustion Engines lab we calculated BSFC (Brake Specific Fuel Consumption) on an old Ford 6 cylinder and found to our surprise (but not the instructors) that the engine made the most of the fuel it burned at wide open throttle. Looking at the power band of the engine and assuming needing 15 horsepower to maintain 55 mph we figured it would get the best mileage in a steady-state cruise with a final drive ratio of something like 1.5:1! Now that was that engine which was pretty much a low end torquer, and it was looking at steady state cruise, not real-world accel/decel/hill/stop kind of driving. When all of that is taken into account Steve's position is closer to the truth. But engine speed does hurt a gas engine.

Diesels don't have throttle plates, so they don't experience the same pumping losses at low "throttle" that gassers do. And I don't have any lab experience with diesels to draw on. But I did have a 2002 F-350 with a 7.3L PowerStroke that used more fuel (measured in dollars) than my '97 with a 460. So I played around with it to figure out whatever I could. It had a trip computer that displayed average fuel mileage. So on one road trip I set the speed control at 55 mph and then at somewhere around 70 mph, in both 3rd and 4th gears (4 speed automatic). I picked the upper speed because it have me the same engine rpm in 4th gear that I had at 55 mph in 3rd gear. Steady state cruise on flat level ground I got about 21 mpg at 55 mph in 4th gear, 16 mpg at 55 mph in 3rd gear, 15 mpg at ~70 mph in 4th gear and ~11 mpg at 70 mph in 3rd gear.

The most amazing thing to me about that was how much more engine speed affected mileage than vehicle speed did. Speeding the engine up by increasing vehicle speed in 4th gear from 55 to ~70 cost me only 1 more mpg than the same increase in engine speed by downshifting to the same higher engine speed and keeping the same vehicle speed.

Surprising but true.

(And no, I didn't get 21 mpg at 55 mph with that truck except in a steady flat cruise. On a highway (not freeway) trip at 55 mph I might average 14 mpg. Overall I was closer to 12~13 mpg, and since it burned more expensive diesel fuel, my 460 at 10~11 mpg is generally cheaper to drive.)

[1986F150Six]

With a gas engine lower vacuum (more open throttle) gives more efficiency, partly because of the reduced pumping loses, the engine doesn't have to work to pull air past the closed throttle plate. Back in a college Internal Combustion Engines lab we calculated BSFC (Brake Specific Fuel Consumption) on an old Ford 6 cylinder and found to our surprise (but not the instructors) that the engine made the most of the fuel it burned at wide open throttle. Looking at the power band of the engine and assuming needing 15 horsepower to maintain 55 mph we figured it would get the best mileage in a steady-state cruise with a final drive ratio of something like 1.5:1! Now that was that engine which was pretty much a low end torquer, and it was looking at steady state cruise, not real-world accel/decel/hill/stop kind of driving. When all of that is taken into account Steve's position is closer to the truth. But engine speed does hurt a gas engine.

Good stuff, Sir!

My son's 1984 F150 with 4.9L, 4 speed manual O.D. and small factory air dam on front bumper was "born" with 2.47 rear gears. [2.47 X .71 O.D. = 1.75]

Before gasohol, this truck routinely returned 26+ mpg @ 55-60 mph.

 

[Ray Cecil]

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.

Factors in mpg:

Torque @ rpm

Vehicle weight

Vehicle speed

Wind direction

Wind speed

Humidity

Air temperature

Elevation

Tire pressure

Tune/timing

Condition of engine/trans

Wind resistance/ drag coefficient

Vehicle height

Tire circumference

Tire tread pattern

Suspension alignment

Condition of bearings

Proper release on brake shoes and calipers

Driver habits

Ac and accessories being engine driven, on or off

Fuel grade/mix

Engine modifications

Engine air/fuel ratio and its devices that control it. (Carb vs injection)

Im probably missing something. However, its impossible to do a proper scientific expirement in the real world on real roads while any or all of these conditions can change. That is why laboratories exist that control all these conditions to isolate each variable. Dave's road tests are good, but I highly doubt he is controlling for all factors. Id say his results are suggestive, rather than conclusive.

[Gary Lewis]

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....

.... 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....

With a gas engine lower vacuum (more open throttle) gives more efficiency, partly because of the reduced pumping loses, the engine doesn't have to work to pull air past the closed throttle plate. Back in a college Internal Combustion Engines lab we calculated BSFC (Brake Specific Fuel Consumption) on an old Ford 6 cylinder and found to our surprise (but not the instructors) that the engine made the most of the fuel it burned at wide open throttle. Looking at the power band of the engine and assuming needing 15 horsepower to maintain 55 mph we figured it would get the best mileage in a steady-state cruise with a final drive ratio of something like 1.5:1! Now that was that engine which was pretty much a low end torquer, and it was looking at steady state cruise, not real-world accel/decel/hill/stop kind of driving. When all of that is taken into account Steve's position is closer to the truth. But engine speed does hurt a gas engine.

Diesels don't have throttle plates, so they don't experience the same pumping losses at low "throttle" that gassers do. And I don't have any lab experience with diesels to draw on. But I did have a 2002 F-350 with a 7.3L PowerStroke that used more fuel (measured in dollars) than my '97 with a 460. So I played around with it to figure out whatever I could. It had a trip computer that displayed average fuel mileage. So on one road trip I set the speed control at 55 mph and then at somewhere around 70 mph, in both 3rd and 4th gears (4 speed automatic). I picked the upper speed because it have me the same engine rpm in 4th gear that I had at 55 mph in 3rd gear. Steady state cruise on flat level ground I got about 21 mpg at 55 mph in 4th gear, 16 mpg at 55 mph in 3rd gear, 15 mpg at ~70 mph in 4th gear and ~11 mpg at 70 mph in 3rd gear.

The most amazing thing to me about that was how much more engine speed affected mileage than vehicle speed did. Speeding the engine up by increasing vehicle speed in 4th gear from 55 to ~70 cost me only 1 more mpg than the same increase in engine speed by downshifting to the same higher engine speed and keeping the same vehicle speed.

Surprising but true.

(And no, I didn't get 21 mpg at 55 mph with that truck except in a steady flat cruise. On a highway (not freeway) trip at 55 mph I might average 14 mpg. Overall I was closer to 12~13 mpg, and since it burned more expensive diesel fuel, my 460 at 10~11 mpg is generally cheaper to drive.)

Bob - I'm jealous of you getting to go into the lab. I didn't take ICE as I was in EE, but a buddy of mine did take it and I bought the textbook from him - and have read it multiple times. But, I didn't get to go into the lab.

Anyway, you make a good point about a diesel not having a throttle, so technically not having "pumping losses". So I was wrong when I mentioned that as an issue since Sideflop's engine is the IDI. But, you did prove that the RPM does make a significant difference, regardless of whether it is pumping losses or something else. Phrasing your data differently, and assuming you had the E4OD with 3.55 gears:

• 55 MPH: In 4th you were turning 1500 RPM and got 21 MPG. But by shifting to 3rd you were turning 2100 RPM and dropped to 16 MPG. That's a 24% reduction in MPG

• 70 MPH: In 4th you were turning 1900 RPM and got 15 MPG, but dropping to 3rd gear you were turning 2700 RPM and got 11 MPG. This is a 27% reduction in MPG.

Granted this wasn't an exhaustive test, but my understanding is that the ECU was doing the calculations based on the amount of fuel it was injecting into the engine, so your load, weather, type of fuel, etc wasn't changing much. Therefore it should be very indicative of the kinds of MPG change that would be seen by changing axle gears.

[Nothing Special]

Bob - I'm jealous of you getting to go into the lab. I didn't take ICE as I was in EE, but a buddy of mine did take it and I bought the textbook from him - and have read it multiple times. But, I didn't get to go into the lab.

Anyway, you make a good point about a diesel not having a throttle, so technically not having "pumping losses". So I was wrong when I mentioned that as an issue since Sideflop's engine is the IDI. But, you did prove that the RPM does make a significant difference, regardless of whether it is pumping losses or something else. Phrasing your data differently, and assuming you had the E4OD with 3.55 gears:

• 55 MPH: In 4th you were turning 1500 RPM and got 21 MPG. But by shifting to 3rd you were turning 2100 RPM and dropped to 16 MPG. That's a 24% reduction in MPG

• 70 MPH: In 4th you were turning 1900 RPM and got 15 MPG, but dropping to 3rd gear you were turning 2700 RPM and got 11 MPG. This is a 27% reduction in MPG.

Granted this wasn't an exhaustive test, but my understanding is that the ECU was doing the calculations based on the amount of fuel it was injecting into the engine, so your load, weather, type of fuel, etc wasn't changing much. Therefore it should be very indicative of the kinds of MPG change that would be seen by changing axle gears.

It was an '02 truck, so not an E4OD. I'm not sure what trans it had, but that's not that important for the overall lessons (and as Ray points out, this experiment wasn't taking nearly everything into account, so that's all there is to gain here: overall lessons). And it had 3.73 gears (again, for what that's worth...).

But the only real correction I'd make to your phrasing is that 3rd gear at 55 mph and 4th gear at whatever speed I was going (somewhere around 70) were the same engine speed on my tach. I was trying to see how much vehicle speed vs engine speed affected my mileage, so I aimed for the same engine speed.

And yes, this was done during a period of maybe 20 minutes as I drove down a pretty flat, straight, boring piece of highway. Wind speed and direction changes were pretty minimal, and essentially no elevation changes so hills weren't a factor. Pretty much no other changes other than vehicle and engine speed.

But I would caution against looking at these big percentages and thinking they might indicate what changes in mileage you could expect with gearing changes. Again, the 21 mpg in 4th at steady-state 55 mpg translated to only around 14 mpg in real life 55 mph driving. While dropping to 3rd at steady-state 55 mph dropped to around 16 mpg, I doubt the real world 55 mph driving like that would have dropped nearly as much, maybe from 14 to 12 mpg or so??? Just saying be careful not to extrapolate this too far. But yes, engine speed does affect fuel consumption.

 

[Gary Lewis]

Bob - I'm jealous of you getting to go into the lab. I didn't take ICE as I was in EE, but a buddy of mine did take it and I bought the textbook from him - and have read it multiple times. But, I didn't get to go into the lab.

Anyway, you make a good point about a diesel not having a throttle, so technically not having "pumping losses". So I was wrong when I mentioned that as an issue since Sideflop's engine is the IDI. But, you did prove that the RPM does make a significant difference, regardless of whether it is pumping losses or something else. Phrasing your data differently, and assuming you had the E4OD with 3.55 gears:

• 55 MPH: In 4th you were turning 1500 RPM and got 21 MPG. But by shifting to 3rd you were turning 2100 RPM and dropped to 16 MPG. That's a 24% reduction in MPG

• 70 MPH: In 4th you were turning 1900 RPM and got 15 MPG, but dropping to 3rd gear you were turning 2700 RPM and got 11 MPG. This is a 27% reduction in MPG.

Granted this wasn't an exhaustive test, but my understanding is that the ECU was doing the calculations based on the amount of fuel it was injecting into the engine, so your load, weather, type of fuel, etc wasn't changing much. Therefore it should be very indicative of the kinds of MPG change that would be seen by changing axle gears.

It was an '02 truck, so not an E4OD. I'm not sure what trans it had, but that's not that important for the overall lessons (and as Ray points out, this experiment wasn't taking nearly everything into account, so that's all there is to gain here: overall lessons). And it had 3.73 gears (again, for what that's worth...).

But the only real correction I'd make to your phrasing is that 3rd gear at 55 mph and 4th gear at whatever speed I was going (somewhere around 70) were the same engine speed on my tach. I was trying to see how much vehicle speed vs engine speed affected my mileage, so I aimed for the same engine speed.

And yes, this was done during a period of maybe 20 minutes as I drove down a pretty flat, straight, boring piece of highway. Wind speed and direction changes were pretty minimal, and essentially no elevation changes so hills weren't a factor. Pretty much no other changes other than vehicle and engine speed.

But I would caution against looking at these big percentages and thinking they might indicate what changes in mileage you could expect with gearing changes. Again, the 21 mpg in 4th at steady-state 55 mpg translated to only around 14 mpg in real life 55 mph driving. While dropping to 3rd at steady-state 55 mph dropped to around 16 mpg, I doubt the real world 55 mph driving like that would have dropped nearly as much, maybe from 14 to 12 mpg or so??? Just saying be careful not to extrapolate this too far. But yes, engine speed does affect fuel consumption.

Bob - Good points. You did say that the instantaneous MPG you were getting was far better than the average MPG the truck returned, so it isn't reasonable for anyone to expect to get MPG's like the instantaneous readings.

However, what you showed is that gearing does matter. Your transmission should have been the 4R100, which had the same gear ratios as the E4OD. So let's redo my rephrasing of your test using the 3.73 gear ratio and same engine speeds for 3rd @ 55 MPH and 4th at whatever:

• 55 MPH: In 4th you were turning 1579 RPM and got 21 MPG. But by shifting to 3rd you were turning 2224 RPM and dropped to 16 MPG. That's a 24% reduction in MPG

• 77 MPH: In 4th @ 77 MPH you'd be turning 2224 RPM, the same as in 3rd at 55 MPH. And at that RPM you were getting 16 MPG. And by pulling it down into 3rd at that speed you were turning 3113 RPM, which reduced the MPG to 11, which is a 27% reduction.

It is interesting, at least to me, to note that the gear ratio change is 29%. In other words, 3rd is 1:1 and 4th is .71:1. So a 29% change in engine speed got a bit less than that in MPG - under steady-state conditions. But under stop and go conditions the overall MPG is far less.

However, if you got 16 MPG at 55 MPH using 4th and it dropped to 12 MPG by using 3rd, that would be a 25% drop. Just saying.....

[Nothing Special]

.... However, if you got 16 MPG at 55 MPH using 4th and it dropped to 12 MPG by using 3rd, that would be a 25% drop. Just saying.....

After I posted that I wondered what percentage the number I arbitrarily picked gave! But I actually only got about 14 mpg in real-world 55 mph driving, so my arbitrary guess at 12 mpg in 3rd is only a 14% drop. But my real point is that it was an arbitrary guess, and it probably would've been significantly less than a 29% drop.

For what it's worth, everyone told me that I should've been getting high teens in real-world mileage with that truck. But I didn't. Based on this test I think the axle gearing was a big part of that. That big turbo diesel could've easily pulled 3.55 gears, if not 3.23 or even taller. I'm sure it would've done better like that. And the shift points for the auto trans were at least 5 if not 10 mph higher than they should've been in normal driving. I tried using a really light foot, or letting up on the accelerator when I wanted it to shift, but there was just no way to get it to upshift until that big diesel was roaring. Loading it down was another place the shift points were terrible. That engine could've easily lugged down in 4th going up a hill on a 55 mph highway, but even unloaded on hills that weren't all that steep it would unlock the torque converter and drop into 3rd. So with as sensitive as the fuel mileage was to engine speed, there was no way to keep the engine speed down in real-world driving. Did I mention that I hated that truck?

[Gary Lewis]

.... However, if you got 16 MPG at 55 MPH using 4th and it dropped to 12 MPG by using 3rd, that would be a 25% drop. Just saying.....

After I posted that I wondered what percentage the number I arbitrarily picked gave! But I actually only got about 14 mpg in real-world 55 mph driving, so my arbitrary guess at 12 mpg in 3rd is only a 14% drop. But my real point is that it was an arbitrary guess, and it probably would've been significantly less than a 29% drop.

For what it's worth, everyone told me that I should've been getting high teens in real-world mileage with that truck. But I didn't. Based on this test I think the axle gearing was a big part of that. That big turbo diesel could've easily pulled 3.55 gears, if not 3.23 or even taller. I'm sure it would've done better like that. And the shift points for the auto trans were at least 5 if not 10 mph higher than they should've been in normal driving. I tried using a really light foot, or letting up on the accelerator when I wanted it to shift, but there was just no way to get it to upshift until that big diesel was roaring. Loading it down was another place the shift points were terrible. That engine could've easily lugged down in 4th going up a hill on a 55 mph highway, but even unloaded on hills that weren't all that steep it would unlock the torque converter and drop into 3rd. So with as sensitive as the fuel mileage was to engine speed, there was no way to keep the engine speed down in real-world driving. Did I mention that I hated that truck?

Sometimes computers seriously hinder attempts to maximize economy. On Blue there's no way to lock it in a higher gear w/o placing it in Manual mode, and then you'd better stay on top of things closely. But, it can be done and you can get deep into the turbo before allowing it to shift.

I'd wondered about doing that on our vacation where we towed the Sea Ray to Lake Powell. In the end I just locked it out of 6th for the most part, save for looooooong downhill runs, and let it do its own thing shifting down from there.

Anyway, at least the newer trucks have that ability. And, with my Mongoose cable and Core Tuning software I can reprogram the E4OD for Dad's truck for shift points, lock/unlock points, etc. Bill's done that already on Darth.