Note, you really need to LOVE the car to do this, it will eat u time and a fair amount of money, but if you are really into keeping the car, you will benefit in the long run.
The only real way to extract more economy out of you car is to change the gearing throughout the drivetrane. This means sourcing a MB 5 speed that will mate up to that engine. Luckily these should be readily available since MB installed them in many cars worldwide.
You will need the Transmission, bellhousing, pedal box, or maybe just a clutch pedal and master cyl. slave cyl, and normal clutch related parts. You may also need a new driveshaft, but again OE MB parts can be had. This is probably out of scope of most DIY types, if you feel comfortable around transmissions etc. then plan on a few days and lots of beer to get you through.
The payoffs can be awesome in terms of BOTH performance and economy. I have not done a MB yet, but I did this about 9 years ago with my Jag XJS V12. These cars only cam with GM400 3 speed autos, no manual trannys offered. I was getting 18mpg at best on the highway, and this was with the US spec 2.88 rear end...not good, let alone this 4500lbs pig was not quick off the line. I custom made a bellhousing and adaped a TREMEC TKO 5 speed in the car. The tremec has a .68 OD or (5th gear).
With the better gearing, the performance of the car improved from 7.8 0-60 stock, to a now 6.1 time. I shaved 2.5 seconds off of the 1/4 mile to a respectable 12.92. And to top it all, I saw and increase in economy to 30mpg!!! At 80mph, this car turns 2000 rpm, barely making an effort.
Now mind you, after all that work, I getting burried in that Jag
