2 weeks ago I posted a question on a rear diff swap and got some good answers, I now have a couple more.
I own 1984 300d turbo cuurently with a 3.07 ratio, I have located a 2.88 ratio diff that I will pick up. I also talked to a local trans guy who does alot of german trans and said that the driven gear would be no problem to change so that the speedo will read correctly.
Today after crunching the numbers, my math sucks, but I think I came up with the correct numbers as close as I can get them. A swap from a 3.07 to a 2.88 rear will give me a 6.25% decrease in rpms. I know already that at 65mph I am turning 3000rpms. so now I will be at 2815rpms approx. and at 75mph I was normally at about 3465rpms I will now go down to 3245rpms approx. It is not much of a gain, though I thought I could do better. my goal is for less noise at highway speeds and at least a 10% gain in fuel economy. I go cross country every so often an looking to get this thing right. next trip is to Denver in late Sept.
Question 1, is it possible to go even lower in ratio as my friends 92' 2.5 turbo has a 2.65 ratio?
are my calculations correct? would it be worth it. does 6.25%rpm decrease translate into a 6% savings in fuel?
any advice is greatly appreciated as this is the premier forum for great answers.
thanks
Peter

You could go to a 1:1 ratio but it will take you about 3 weeks to get up to speed. You will porbably get 100 mpg once you get there but the first small head wind you encounter will drop you back to 3rd. gear or less. There is a point of diminishing returns here where the graph for the ultimate mpg will cross the lack of acceleraton just about the time you pull out to try and pass that semi on that 2 lane with a slight up hill grade some hot afternoon with the sun in your eyes. The question is; is that extra 2 mpg worth it?

Your calculations are correct, and yes it is possible to use taller gears than the 2.88 gears that you currently have, however, it is probably not in your best interest to do so.

The problem occurs with horsepower. The substitution of 2.88 gears has reduced your available horsepower by 6.25% at a given speed. You will notice, out on the highway, that pressing down on the pedal at 65 mph yields noticeably less response than what you originally had with 3.07 gears.
However, and this is important, the required torque from the engine has increased. Why is this? Well, the required torque to the pavement has remained the same at a given speed and the available torque to the pavement has decreased by 6.25% because of the gear ratio. So, the only alternative is to increase the torque from the engine by 6.25% to compensate. So, with the increased torque from the engine comes higher exhaust temperatures across the operating range.

This is the main reason that you will not get anywhere near a 6.25% increase in fuel efficiency by switching to the 2.88 gears. The engine has to work harder to push the vehicle at 65mph and it requires additional fuel to do this. Now, there are some benefits of running the engine slower due to reduced frictional losses within the engine. You need less fuel to run the engine at 3000 rpm than you do at 2700 rpm, all else being equal.

I can tell you my personal results of switching from 195-70-14 tires to 215-70-14 tires. The reduction in engine speed is about 3.5%. I could not measure any difference in fuel economy after the odometer was properly corrected for the larger tires.