240D differential swap
 

This has been covered frequently even by me. I now have my final definitive report. My 240 D has 195x70x14 michelins and a MT. With the stock 3.69 dif it performed about as expected with a nice granny first gear and balanced shifting. It 65 the engine was turning pretty high, at 75 ridiculously high esp compared to my 85 300D with an AT.. I found a low mile 3.07 following the advice given by a fine well known mechanic in Bellingham WA. Pros: extremely quiet low revs at highway speeds but getting there took a while. Better economy 37+ with my new injectors compared to 35-36. Cons: needed a lot more clutch esp staring on soft ground. Frequent stalls. Lost speed going up even slight grades. Much diminished acceleration. I had some 185x60x14 tires from my old corolla and decided to put them on the back. This had the effect or changing the effective FD Dif from 3.07 to about 3.44. Pros: much improved standing starts with minimal clutch needed. Better acceleration . Cons: the diameter is about 2" less compared to 195 tires so the car sits a little lower, an inch or so but I cannot go back to the 3.07 ratio. Conclusion the 3.4+ is the ideal ratio for the 240D. I am pondering putting in a 3.46 which would allow me to have taller tires. I wonder if putting in a 3.46 ring gear would be easier. Swapping out these difs is a simple job and worth doing esp if you have boots or dif seals, flex couplings to do(I don't). Should I just be happy with the little tires?

maybe. nothing wrong with them though except the speedo being off.

This is my next project. I have an '81 240D, 4 speed manual. Sitting in my garage is 3.46 differential and matching speedometer. The factory axles will go to CVJ in Denver. New flex discs and rear suspension rubber. The car kind of shows its lack of love from previous owners. But the engine is a 20 year old factory rebuilt with almost 400psi compression across all four. I wonder if I will outlast the car or vice versa.

Final Drive Ratios
 

Your having a manual trans. makes a difference too .

I forget what the diffy in my 82 240D's ratio is, I agree that it's buzzy above 65 MPH .

IIRC the original tires size is 175 width.....

I have driven the car very hard with these skinny tires and I ran out of brakes before the car ever lost traction going down the mountain near Porterville, Ca. .

Have you tried a set of light alloy wheels with your car?

Light Alloy Disc Wheels
 

I have and to me they're better, the car sticks better on uneven pavement and the slight reduction in rotating mass allows my little 2.4 to accelerate a bit better .