om603 5 speed conversion: differential swap
 

Hey all,

Between May and June 2018 I swapped a .412 transmission from a 190 2.6e into my 1987 300 TD (om603 engine). Unfortunately, I deployed to the Middle East before I had a chance to fine tune everything, so I never really tested it out. I'm still here through the spring.

Question: which replacement differential will give me the best combination of acceleration, fuel economy, and highway cruising? I searched old threads and the consensus seems to be either a 3.07:1 or 3:27:1 diff, but with time maybe the consensus has gravitated to one ratio over the other.

My intention with the car is to make it a reliable and comfortable commuter (yeah, I know it's north of 30 years old ...).

Many thanks,

Corvo Gold
-'87 300 TD

I put a 3.07 in my 5spd swap about 6months ago after 5 years of driving it with the 2.65. I think that's the best compromise of acceleration and fuel economy and quiet highway driving. 2.65 with a loaded car and AC running made for some interesting hill starts. The 3.07 also made the car a lot more fun to drive. The over all gearing in 5th is still about 7% taller than the stock 4spd auto.

1) The 717.412 trans has a 1st gear of 3.91:1, and a 5th of .77:1.
With a 3.27:1 final drive, 1st overall will be 12.79:1 which is more that low enough for the OM603 to pull off from a stop comfortably.
2) At .77:1 and 3.27:1 in 5th, the overall ratio will be 2.52:1, which is likely to be just right for 70-75 MPH cruising. (By way of comparison, the V8 W126 cars had a 2.47 overall ratio.)