Rear end gearing

[Gary Lewis]

The higher the compression ratio (to a point) the more efficient an engine is.

I've never heard that one, or done any calculation that would suggest it. But basic entropy graphs show that about TEMPERATURE - the higher the engine's peak combustion temperature, the more-efficient it is (thermodynamically).

Your quote implies that the cycling (rotating mass of the engine components) is the greatest loss, and that an engine with no moving parts (like a ramjet) must necessarily be the most-efficient. A ramjet is certainly thermodynamically-efficient, but it's not fuel-efficient (MPG), which is what we're really talking about.

And all things being equal, the most-efficient mechanical engine is a Stirling, which has very low compression ratio.

It takes energy to pull air past a closed throttle plate.

I disagree strongly with that...

Fluids can't be "pulled" - only pushed. And the engine doesn't push the air outside - the earth's gravity does that. The engine has to push the air in the crankcase out of the way for the piston to go down. That's irrelevant of the throttle or its size. But an open throttle allows air to flow into the engine, balancing out the pressures above & below the piston, making it easier to pull the piston down.

That might seem semantic, but it's a physical fact: the engine uses NO energy to move air past a closed throttle plate. Only to push the air around inside the crankcase, to compress it into the combustion chamber (and higher CR means more loss there), and to push it out of the cylinder during the exhaust stroke.

Vehicle efficiency is what we most often care about: how far can I get on a certain amount of fuel.

That's what the MythBusters & Top Gear experiments were about (although the TG one was intentionally slanted to produce the result they wanted, for entertainment reasons).

...try to squeeze every last 1/10th of a mile from it. Id rather drive it like I stole it...

That has always been my philosophy when I'm harassed about driving such a large, heavy, vehicle getting such low MPG; I'd rather pay for that extra gas than lose the safety, durability, & utility.

I'm not trying to shut down the discussion, but Sideflop may be getting confused. So to cut to the chase, regardless of what was said in engineering classes or what we can read on line, the bottom line is that gearing does matter. Bob proved that for us with his test of different gear selections in the same truck, with a diesel engine, on the same day in the same weather conditions. To reiterate, his instantaneous readings, given by the vehicle's ECU, were:

• 55 MPH: In 4th the engine was turning 1579 RPM and got 21 MPG. But by shifting to 3rd it was turning 2224 RPM and dropped to 16 MPG. That's a 24% reduction in MPG for a 29% increase in RPM.

• 77 MPH: In 4th @ 77 MPH it was turning 2224 RPM, the same as in 3rd at 55 MPH. And at that RPM it was getting 16 MPG. But by pulling it down into 3rd at that speed it was turning 3113 RPM, which reduced the MPG to 11, which is a 27% reduction in MPG for a 29% increase in RPM.

Granted, those were not the average MPG's he got with the truck. In fact, they are much higher than what he got. But they indicate that changing the engine's RPM w/o changing anything else does change the vehicle's economy.

And then back to the original question:

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?

To that I'll say again essentially what I said earlier to answer that question:

• "15-18 mpg going 65-75 mph" is pretty doggone good for a heavy truck, and I'd be delighted with it

• 3.54 is a decent ratio for towing as well as cruising light since it has the engine at an RPM where it has the torque to tow in 4th gear

• However, the limiting factor is the lack of an overdrive gear in the transmission. If a ZF5 were to replace the T19 then you could tow in 4th just as you do today, but when running light shift into 5th and enjoy better MPG as well as reduced engine noise due to the reduced RPM's.

Note that I'm not quantifying the increased MPG. But a 24% reduction in RPM will yield some increase in MPG.

[Ford F834]

I'm not trying to shut down the discussion, but Sideflop may be getting confused. So to cut to the chase, regardless of what was said in engineering classes or what we can read on line, the bottom line is that gearing does matter. Bob proved that for us with his test of different gear selections in the same truck, with a diesel engine, on the same day in the same weather conditions. To reiterate, his instantaneous readings, given by the vehicle's ECU, were:

• 55 MPH: In 4th the engine was turning 1579 RPM and got 21 MPG. But by shifting to 3rd it was turning 2224 RPM and dropped to 16 MPG. That's a 24% reduction in MPG for a 29% increase in RPM.

• 77 MPH: In 4th @ 77 MPH it was turning 2224 RPM, the same as in 3rd at 55 MPH. And at that RPM it was getting 16 MPG. But by pulling it down into 3rd at that speed it was turning 3113 RPM, which reduced the MPG to 11, which is a 27% reduction in MPG for a 29% increase in RPM.

Granted, those were not the average MPG's he got with the truck. In fact, they are much higher than what he got. But they indicate that changing the engine's RPM w/o changing anything else does change the vehicle's economy.

And then back to the original question:

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?

To that I'll say again essentially what I said earlier to answer that question:

• "15-18 mpg going 65-75 mph" is pretty doggone good for a heavy truck, and I'd be delighted with it

• 3.54 is a decent ratio for towing as well as cruising light since it has the engine at an RPM where it has the torque to tow in 4th gear

• However, the limiting factor is the lack of an overdrive gear in the transmission. If a ZF5 were to replace the T19 then you could tow in 4th just as you do today, but when running light shift into 5th and enjoy better MPG as well as reduced engine noise due to the reduced RPM's.

Note that I'm not quantifying the increased MPG. But a 24% reduction in RPM will yield some increase in MPG.

As interesting as all of the science behind fuel efficiency is, I wanted to sum up what I have gleaned from years of hanging out on IDI forums. Owners of 6.9/7.3 Report the best mpg with a final drive ratio of about 2.75-2.50, go much above that and you don’t gain much, if anything, at the pump, but may have higher EGT’s and less ability to hold gear on any kind of grade. In a 8 lug truck, 3.55’s are about the highest axle gears you will find. 3.55 (Ford tooth count) 3.54 (Dana tooth count) with a ZF-5 gives you a final ratio of 2.73. This is just about perfect for the 6.9 and stock wheels and tires. It will give you 20+ if you keep your speed down around 60-65 and your IP and injectors are healthy and in time. An auxiliary overdrive behind the T19 will accomplish the same thing, but unless you find a used one it will cost more than a ZF swap and is clumsy to use. The diesel T19 already has really close ratios so splitting gears is redundant and not a benefit. I have one and I’m not crazy about it. I need that and a ZF to overcome my 4.10’s. My 6.9 pulled the strongest around 1800-2000 rpm’s and seemed to get the best mileage there. Mine (crew cab 4x4) got 17 mpg with a final ratio of 3.20, so if you are hitting 18 with 3.55’s you are right there with what can be expected. In terms of dollars and cents you will probably never recover the cost of a ZF swap in fuel savings, but in pleasure of driving it will pay for itself in the first mile.

[Gary Lewis]

As interesting as all of the science behind fuel efficiency is, I wanted to sum up what I have gleaned from years of hanging out on IDI forums. Owners of 6.9/7.3 Report the best mpg with a final drive ratio of about 2.75-2.50, go much above that and you don’t gain much, if anything, at the pump, but may have higher EGT’s and less ability to hold gear on any kind of grade. In a 8 lug truck, 3.55’s are about the highest axle gears you will find. 3.55 (Ford tooth count) 3.54 (Dana tooth count) with a ZF-5 gives you a final ratio of 2.73. This is just about perfect for the 6.9 and stock wheels and tires. It will give you 20+ if you keep your speed down around 60-65 and your IP and injectors are healthy and in time. An auxiliary overdrive behind the T19 will accomplish the same thing, but unless you find a used one it will cost more than a ZF swap and is clumsy to use. The diesel T19 already has really close ratios so splitting gears is redundant and not a benefit. I have one and I’m not crazy about it. I need that and a ZF to overcome my 4.10’s. My 6.9 pulled the strongest around 1800-2000 rpm’s and seemed to get the best mileage there. Mine (crew cab 4x4) got 17 mpg with a final ratio of 3.20, so if you are hitting 18 with 3.55’s you are right there with what can be expected. In terms of dollars and cents you will probably never recover the cost of a ZF swap in fuel savings, but in pleasure of driving it will pay for itself in the first mile.

Well summed, Jonathan.

I particularly like your last sentence, and fully agree. The reduced engine noise because the RPM at 65 MPH on Dad's truck went from 2800 to 1900 was well worth the cost of the swap.

And that doesn't even address the 2 MPG hoped-for increase in MPG due to the T19 to ZF5 swap on Big Blue. Currently Big Blue gets 11 MPG, so a 2 MPG increase gives 13 MPG. Assuming gas is $3/gal my math says it'll take 37,750 miles to make the $1500 cost of the fully-rebuilt ZF5 up. I expect to drive BB that far, so I suspect I'll save money eventually with the swap. However, the cost of the ZF5 was a one-time expenditure and filling BB up is a very frequent event, and it is that frequent reminder of the very poor MPG he currently gets that is galling.

Can you tell I'm looking forward to getting the ZF5 in?

[Steve83]

...instantaneous readings, given by the vehicle's ECU...

Was the ECU reprogrammed for the change in gears?

...a 24% reduction in RPM will yield some increase in MPG.

Not necessarily, because you're ignoring the engine load. An engine screaming along at redline in Neutral doesn't burn nearly as much as one barely holding 1000RPM going up a hill towing its GCWR in OD. That's the extreme example, but the same thing happens to a lesser degree when the transmission is shifted up, or the drive axle gears are lowered (numerically higher), or the tire size is increased, or...

[Nothing Special]

...instantaneous readings, given by the vehicle's ECU...

Was the ECU reprogrammed for the change in gears?

The truck was running stock gearing, so no change needed. But even if the gearing had been changed, all of that testing was done over less than 30 minutes in a single road trip, so very few variables between runs.

...a 24% reduction in RPM will yield some increase in MPG.

Not necessarily, because you're ignoring the engine load. An engine screaming along at redline in Neutral doesn't burn nearly as much as one barely holding 1000RPM going up a hill towing its GCWR in OD. That's the extreme example, but the same thing happens to a lesser degree when the transmission is shifted up, or the drive axle gears are lowered (numerically higher), or the tire size is increased, or...

Not ignoring engine load, specifically testing for it. 55 mph in 3rd and 4th, 4th gave much better mileage (21 mpg vs 16). 70+ mph in 3rd and 4th, 4th gave much better mileage (15 vs 11 mpg). So no change in load, only changing engine speed made a big change in fuel mileage in that truck.

And even when changing load the mileage hardly changed. 70+ mph in 4th gear was the same engine speed but much higher load than 55 mph in 3rd, and the mileage wasn't much different (15 vs 16 mpg).

Again, this was one real-world test with one truck, a 2002 F-350 with 7.3L PowerStroke, 3.73 gears, stock tire size (~32" OD?) and a 4 speed auto trans. All tests were about 2 miles of flat, level steady cruise, with the torque converter locked. As they say, your mileage may vary:nabble_smiley_beam: