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Here is a little formula to calculate the MPH at a certain rpm.
1/(final drive ratio) x (rpm) x (X,this is variable) = MPH
Here is one that I have done
1/2.24 x 2500 x (X) = 78 mph
X = .069888
The final drive of our 400E/E420s is 2.24 so that is the fist one. The rpm of 2500 at 78 mph is what I have observed while driving. So by replacing variables, you can change the gear ratio and find the engine speed at any given rpm.
I have been thinking about this lately and I came up with this formula to find out how livable a new rear end would be. I have come to the conclusion that a 3.07 is to short for my driving. I travel on the highway a lot, so my gas mileage would go down to much, and I calculated that at 60 mph my rpm's would be at approx. 2600 and that is to much for me (now it is below 2000). I would love a new rear end and with gas prices falling (knock on wood), who knows it might happen. If I would only drive around in the city I would be more viable to me. I don't want to give up my low reving highway killer. Thirty five hundred rpm at 80 mph is a lot my car now is around 2700.
Oh yea dont forget the extra ware and tare on the motor.
Subman
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