Well the engineers had their parameters.

any design whether it be a building a whizzer motor bike or our revered mercedes benzes is a combination of compromises.

In my car the only explanation for the top speed being the same in fourth or fifth is that as the RPM rises so does the horsepower but only up to a point. The torgue is no doubt developed at a lower rpm.

The amount of power available to push the car through the air is constant if the air density and so forth is constant. So in fourth for example the hp is probably a bit more at lets say 4500 rpm and the torque a bit less.

In fifth the od is about 18% so the rpm will drop maybe 800 at which point the torque probably dominates the hp a bit and it all balances out.

Brian always likes to argue with me about my fuel mileage experiences but his experiences are with gas engined chrysler products from 30 years ago so as it relates to diesels I believe he mainly has his formulas to argue. I have changed the gear ratios in probably a dozen vehicles and my findings are pretty consistent. He'll say my methods are not scientific but I have kept records for the last 20 years in all my vehicles and I will stand by them.

In these MB diesels and in my suburban diesels if you drop the engine speed 20% you realize nearly an exactly corresponding increase in fuel economy.....20%.

It is not as straightforward with gassers. the economy has never fallen when gearing them "higher" but it does not always correspond as precisely as with the diesels.

If Brian has changed his gear ratios I will be happy to stand corrected.

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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.

Obviously, you haven't paid any attention when I changed the diff on the SD from 3.07 to 2.88. The fuel economy benefit was about 1 mpg...........hardly the 6.8% that you would claim for such a change.

Others have changed from 2.88 to 2.47 and noticed no change in fuel economy.

So, I'll take your stand as corrected. Diesels don't benefit from the percentage change of the ratio. I'm sure yours don't either but we've had that discussion already.

Hmmm, lets see. 307 / 2.88 = 1.065.

Lets say you were getting 27 mpg highway x 1.065 would equal 28.77. That is less than 2 mpg.

What did you find as your results?

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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 already explained that I got about one-half of that amount, as predicted.

Yes, but you thought that 288 divided by 307 was a 9.6% od effect.

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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.

Calculation error...........since corrected.

The fuel benefit was 1 mpg.........about 1/2 of your predicted 1.7 mpg based upon the ratio.

This benefit holds true for most vehicles that undergo the diff change...........except yours. You manage to violate all laws of physics by getting the exact fuel economy percentage as the ratio change. Magically, the force required to dispel the air has also been reduced by the same differential percentage on your vehicles. Additionally, the work required to accelerate the vehicle around town is also reduced by the same differential percentage on your vehicles.

This statement is telling..............you have no understanding of horsepower and torque and the relationship between the two.

I think you overstate. I know what my car will do. Come on out to Indiana and I will give you a ride.

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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.

Yes, I "overstate"...........others think the same..........I'm an "overstater".

Guilty.

Actually I have an excellent understanding of the two.

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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.

Of course you do. That's why you refer to torque as a parameter in top speed results.