Sure there are many factors to consider. But, the claim is that he can get exactly the same fuel economy improvement as the axle ratio change.

This is a fundamental physical impossibility unless the engine speed is the only variable.

Since the vehicle requires a fixed amount of fuel to push it thorough the air, he'd need a much larger benefit from the engine speed reduction than the 20% he got from the axle ratio change.

The fuel savings is not going to be great enough to offset the constant fuel consumption required to push the vehicle.

Fundamental physics........light foot......heavy foot......doesn't make a bit of difference. You cannot get a fuel economy benefit of 20% from an axle change of 20%. It's impossible.

After thinking about it a bit more, there is one scenario where he might get the full 20%. It would require a very large SI engine that runs at high rpm's and requires a huge amount of fuel to keep itself running at high rpm's, whether under load........or not.

If you reduced the rpm's by 20% on such an engine, you would save more than 20% of the fuel required to simply rotate the engine under no load. You might save as much as 40% of the fuel required. If the difference in fuel consumption between the 20% saving and the 40% saving would exactly match the additional fuel required to push the vehicle through the air, then he would save the 20% that he is claiming.

But, such a scenario is impossible on a diesel engine, because the fuel required to keep the engine running at high rpm's is still a fraction of the fuel necessary to push the vehicle. Reducing those rpm's certainly reduces the fuel required at no load. But, the savings at no load cannot compare to the significant fuel required to move the vehicle. Therefore, the savings on a diesel engine, and most smaller SI engines, cannot equal the percentage drop in the engine rpm's.

you seem to think my methods of scientific observation are basically.....dog doo doo. exactly what methods do you use to calculate fuel mileage... and i dont mean in the chrysler engineering department, i mean in the real world?

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Basically correct.

I've determined another reason for your failure to provide good data.

When you install the taller axle, you naturally drive the vehicle "slower" because it feels "slower". You don't push it as hard and accelerate in the same manner as when the shorter axle was installed. I'd also take a guess that you don't achieve the same highway speeds with the taller axle........don't want to push the engine that hard.

Your example of how you fuel the vehicle is filled with potential errors. Letting the pump shut off leaves an unknown quantity of air at the top of the tank. The proper way is to fill it to the cap........every time......even though it takes longer to accomplish this and you must wait for the foam to reduce.

As mentioned above, your data from a high powered SI engine with a 20% reduction in axle ratio may be correct. But, you have decided to extrapolate this result to all your vehicles and you have no data to support it.

Brian:
I'm with you on almost everything you've said, except for the air at the top of the pipe statement. I fill my diesel vehicle to the tippety top, too. But I do it not because I need to in order to accurately calculate my mileage....which I have done for years and years. I do it because I hate stopping for fuel . And I'm an anal retentive idiot.
Tom knows he can put any random amount of fuel in his tank....half full, tippety-top full.....78 and 1/4% full....it makes absolutely NO difference when calculating fuel economy. All he needs to know is total gallons in and total miles driven. Remember, he's talking about AVERAGE fuel economy calculated over at least several months, and more likely over several years. As long as he keeps a running total of gallons burned and miles driven, and he does, then he has a very accurate measurement.
There's no reason not to believe Tom. He's simply relaying his observations. I'm a big fan of physics, like you, and I agree with what you are saying, as most people hopefully do. Obviously, by default, Tom's 'go pedal' behavior would change significantly when the gear ratio is changed, so it's not just one variable (the gearset) in play. Tom himself would be a multitude of variables in this situation

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Brian
1995 E320 wagon

there is only one way to drive a diesel whether it is a 240 300 or 300 turbo. push it to the floor and go.

my highway driving with the 240 with 300 motor was about 3 mph more on the highway.

you can stand there and stuff your tank if you want but i am not interested in spilling it on my self for a fraction more fuel in the tank.

and yes of course i keep a running log, recording every tank, and every repair and note where i bought the fuel and what kindof driving i was doing.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I completely believe that he feels his data is accurate. I don't, for one second, think that he is here to mislead the forum.

However, as is common with many statements here, by folks who are not sticklers for data, he provides information that he feels is accurate, but won't stand up to scrutiny from engineering principles. My guess is that his data is perfect for his large displacement gassers that suck up fuel like crazy just to keep themselves running at 3000 rpm.

He then concludes that all engines, smaller SI engines and diesels will provide the same results, and his data is skewed to allow him this conclusion. He also has no solid back to back tests, without any other introduced variables, so that his conclusions are basically flawed because of this.

I agree with him on his large SI engines, but, unfortunately, none of my previous data from Chrysler (all 318 and 225 engines) or any of the data from the members of this forum can agree with him on smaller displacement SI engines or diesel engines. The engineering principles back me up on this.

first which car would you classify as a large displacement gas guzzler that i listed?

and i clearly stated that these were my observations about my experiences. you are the one who is trying to impose your conclusions on other folk's situations.

and what methods did you use to measure the fuel mileage on your sd? other than stuffing the tank?

now the 225 and 318... these are engines i have experience with. of those two gas guzzlers, the king (for its size) is the 225. my 63 valiant was lucky to break 15 mpg on the highway and its speedo was nearly 15% optomistic! so that meant that the fabulous 50,000 mile warranty was gone at a little more than 35,000 actual miles!

actually for its day the 318 (mine was a 65 belvedere) was not too bad on gas. but they never ran right til they were good and warm, right brian?

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Well guys, I don't have any data at all, but my gut says (all things being equal) that the mileage improvement has to be significantly less than the change in gear ratios. Regardless of the ratio, it takes a fixed amount of power (at the wheels) to drive the car at a fixed speed. Changing the ratio only changes the engine speed, not the power required at the wheels (or at the engine output shaft). The fuel consumed is a just function of the engine power required and the efficiency of the engine. Therefore, the only reason that a higher ratio results in higher mileage is the increase in engine efficiency. Running the engine at lower rpm will increase efficiency because the internal friction (and the auxiliary loads) will be less, and the combustion process may be more efficient at that speed (closer to the peak torque rpm, I suspect). However, I find it hard to imagine that a 20% change is ratio would increase the mileage by more than 5% or so. Even a 5% increase in mileage would imply that about 25% of the fuel is being used just to overcome internal engine losses and auxiliary loads, even that sounds too high to me. Just my opinion.

i am not a scientist.

but i have my experiences which i have related.

smokey yunick said that a gas engine uses about 14 parts air to one part of fuel. now if you are turning that engine over on the highway at a given rpm you are pumping x amount of air and fuel mixture through it. FOR IT TO RUN WITHOUT PINGING ETC it will consume (if it is a five liter for example) five liters of air fuel mix for every second rotation. now if you install a higher gear it will turn less rotations per mile. your statement would be true if it were running at optimum effeciency. we all know it is not anywhere near that in most cases. take equal cars with the same gear ratio and same tires and install in one a 2.5 liter and the other a 5.0 liter and unless one of them is actually optimized the five liter one is going to about use twice the fuel.

and in my expereince it works about the same in diesels.

look carefully at my posts regarding the 240ds with 2.4 liter and 3.0 liter motors. one with the 20% larger motor but 20% higher gear and they got equal mileage. the 300 actually did a little better. i suspect because the 240 was turning too high a rpm and was above its optimum operating speed.

saying it takes a specific fuel amount to push the car a certain speed is only true if the engine is at optimum fuel effeciency. they seldom are.

that is why one of the posters can get such excellent economy with a corvette. it has a six speed and plenty of torque to pull a very tall final gear ratio. 1600 rpm for a five liter will use the same as a 2.5 liter running 3200, more or less.

now i think with your turbo motors with the automatic alda adjustment you are getting into a more complex situation. and when you are at a mile elevation like craig then the turbo motors really come into their own for economy.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

It takes a specific amount of fuel to push a car at a certain speed with a given engine no matter what the efficiency, no matter what the gear ratio.

Sooner or later, you'll realize that you don't understand the physics of this discussion.

I refer back to your original statement in post #24. It's accurate.

Tom,
I agree with everything you just posted. Higher ratios will improve mileage for all the reasons you've stated. I was simply saying that the improvement in mileage should not be proportional to the increase in the ratio, it should be less. There may be other factors that I'm not aware of that are affecting the mileage you've observed.

My '63 Corvette has a 3.08:1 axle and achieves 20-22 MPG at typical highway cruise speed. The standard axle ratio with the Special High Performance engine was 3.70 and with that gearing they could achieve high teens, so my 20 percent taller gear yields about ten percent better highway fuel efficiency with revs at 60 of 2340 and 2810, respectively.

The same long block was also available with Rochester FI and because of even fuel distribution, they could be run quite lean and achieve ten percent lower fuel consumption at highway cruise than the carbureted SHP engine with the same gearing.

So if I had FI on my car and set it up right I could probably knock on the door of 25 MPG at highway cruise.

Typical SI engines operate most efficiently at or slightly below the torque peak at low manifold vacuum, but this is complicated by the fact that low manifold vacuum usually enables fuel enrichment on typical carbureted vintage engines, which adds up to about 20 percent more fuel. On a modern car, like our Mercs, full load enrichment is not achieved until you floor the throttle and close the WOT signal switch.

Also, carburetors, especially on pre-emission engines did not maintain very precise fuel metering, and due to distribution issues they must run overall a little richer than with port injection to keep the leanest cylinder from misfiring, so fuel consumption with a carburetor is highly dependent on setup.

The spark advance map is also critical, but many older SI engines do not achieve optimum spark timing at low speed, high load to ward off detonation.

Modern engines maintain a stoichiometric ratio under all, but WOT (and cold start/warmup) conditions. Stoichiometry is driven by EMISSIONS requirements, not fuel economy. Best fuel economy is achieved at about 10 percent lean and this is how the old Rocherster FI systems could be set up, but this increases peak combustion temperature, which considerably increases NOx production during combustion.

Modern engines also have more flexible electronically programmed spark advance maps and they are usually setup to provide near optimum spark timing under most load conditions since they have detonation sensors and algorithms to reduce advance if detonation is detected.

So there are many variables in the fuel efficiency equation.

A short geared car with a large engine operates very inefficiently at highway cruise speed because internal friction and pumping loss are in the same ballpark as road load power.

Though a SI engine may achieve best brake thermal efficiency under the conditions I described above, it has been variously estimated that the overall achieved brake thermal efficiency of a typical car engine in normal driving is about ten percent, which makes them about as efficient as the best railroad steam locomotives of yore.

Pumping loss is essentially eliminated in a diesel engine, so a diesel will not be as sensitive to fuel efficiency delta from gearing changes, and especially on naturally aspirated diesels you can quickly "undergear" them to the point where performance falls from poor to abysmal.

If you replace a 240 with a 300 you have added one more cylinder of the same size, so mechanical friction goes up accordingly. Gear it 25 percent lower and it's a wash in terms of both performance and fuel consumption, so what's the point?

Duke

That's interesting, and it's consistent with a 20% ratio change resulting in a 10% mileage increase. What do you think the percentage of internal loses are for a 617 or 616 at highway speeds, say 3000 or 3500 rpm.

I'm in agreement with everything you note.......as usual.

However, please comment further on the short geared, high displacement SI engine.

If this engine were operating in neutral at a given rpm, and the fuel consumption was measured in gallons per hour........and the engine rpm was then reduced by 20% and the fuel consumption measured again.......would it be your expectation that the drop in consumption would be significantly greater than 20%?

If the engine would drop 40% in consumption from its own internal friction, then, when the constant fuel consumption of the road load was added, it would be perfectly reasonable to expect a 20% drop in fuel economy from a 20% taller rear axle.

This presumes, from your original statement, that the fuel consumption due to internal friction is approximately equal to the fuel consumption caused by the road load.

The only scenario that I see this situation being realistic is for this large displacement SI engine with short gearing. All other scenarios have the road load significantly greater than the internal engine friction.

My best guess on the diesels is that the internal friction of the engine represents 30% of the fuel consumed and the road load of the vehicle represents 70% of the fuel consumed.