I made a engine swap in my MB W124 300E. The new engine is a diesel. It runs smooth. However, it is lazy climbing. My friend said the diff is "far too long". It's final ratio is 3,07 and I want to change it.

There are many ratios to choose from: 3,27 until 3,92.

The car shifts from 4th to overdrive at 133 km/h (about 82 miles/h), at 2500rpm. Too long in fact!

Can any expert say which would be the ideal ratio to get a shift at say 100km/h or 60 miles/hour at 2500rpm?

My calculations (making a linear proportion) says I need a final ratio of about 3.5. Am I right? Think there are MB diffs of 3.46 and 3.67. Do someone know where can I get one? It should fit into the W124.

Second: how can I estimate what would be the diesel consumption after making the diff swap, compared with present 40mpg on the highway?

Thanks a lot ..