What rpm's are you turning at 60mph?
 

2800 rpm at 60mph on a 1987 260E.....looking for a 2.42 differential to drop rpm's....anyone know what year and models will work? I believe a 1975 300D is 2.42, so what gear ratio is in a 1978 300D?

About 3000 in my 300SE. Less in the CVPI but it has no tach, yet.

My 87 300D turns the tach at about 2600 rpm at or about 60 mph.

You may be interested in this formula:

(MPH)(Ratio)x336÷Diameter=RPM

MPH=Speed of vehicle
Ratio=Differential gear ratio
Diameter=Diameter of tire

The result, in RPM, is the engine speed for a manual transmission, or an automatic with lockup, with no slip. You need to add about 200 rpm to this figure for operation of a vehicle with an automatic (without lockup) on level ground.

BTW, the formula is for vehicles with a 1:1 transmission ratio. If you want to use the formula for other gears, you need to multiply the diff ratio by the gear ratio and substitute that value in the formula for "Ratio".

2150 rpm. 97 C280.

.83 5th gear, lockup torque converter, and 3.07 differential.