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Originally Posted by t walgamuth
but in actual practice, especially in a diesel, i have found that if fitted with a 20% od situation you will realize a 20% reduction in fuel use. my conclusion is that the engines are geared with so much reserve torque and power that they are virtually ALWAYS running at much less than their optimum fuel consumption mode to make them more drivable for the average situation for the average driver in the average weather conditions in the average country with the average..... you get the idea.
this is my experience, if any of you have read all this, i am impressed.
tom w
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You will never get a 20% reduction in fuel use from a 20% OD situation on a diesel engine. This has been proven by every member of this forum who changed from a 3.07 to a 2.47 on the SD. The best that any of them can document from this 24% change is about 10% or so.
In my specific case, dropping from a 3.07 to a 2.88, a 7% reduction, resulted in about a 3% fuel economy gain.
The problem with your theory is that you forget about the work that engine must perform, no matter what axle ratio is in the vehicle. A diesel engine must burn a specified amount of fuel to do a specified amout of work........and all your arguments in the world can't get around this little bit of physics.
The only benefit that you receive from a taller axle ratio is reduced engine friction based upon the slower engine speed. It takes less fuel to turn the engine at 2600 rpm than it does to turn it at 3000 rpm. However, this portion of the fuel consumption is probably 30% of the total fuel consumption on the highway. Most of the fuel is consumed to push the vehicle through the air..........and you can't produce any magical energy by changing gear ratios.
Change differentials until you are 85 years old.........but.......don't challenge the engineering of a diesel engine.........the physics speak for themselves.