I put a 2.88 in my '81 300D, but... I have a 4-speed gearbox and have much better control over the rev's and when I shift. My car has some mod's that the P.O. installed.

1. Garrett T04 turbo - it works but this is/was a non-turbo engine but it has plenty of power when you hit the power band.

2. 3-inch air intake with a cone filter. This is getting removed and replaced by a stock filter, housing from a W126. I just have to make an adapter for the larger turbo intake.

I take the car on 3 to 5-hour trips, at interstate highway speeds, on a regular basis. At highway speeds, there is a 15 MPH difference between what is indicated on the speedometer when compared to the GPS. 60 mph indicated = 75 mph GPS. This translates to 33 mpg (NO A/C Running) and 29 mpg with the A/C.

I did install a complete engine coolant system from a W126 for a little extra capacity so temps are not an issue. One day I will to a top speed test... maybe...