leathermang, yeah like JB3 said it should be an overdrive 'box. That's what I was going for.

Here's my original reasoning for wanting lower RPM;

These cars were built for the American market at a time the speed limit was 55 mph. Now I know that Mercedes engineers knew what they were doing, and I know that driving 65-70 won't destroy these engines, but my car's engine, and a lot of others, were built with the intention of running at 55 mph, which is 2623 rpm (26 inch tires). With a 5-spd 5th gear ratio of 0.83 at 65 mph, the rpm's are at 2573. So not only can I go 10 mph faster and get the same rpm, I actually get 50 rpm lower. That was my goal. Cruise at a lower rpm at the same speed. Now that some of ya'll are saying the 616 with its mighty 65(ish) hp wont pull the car at 2573 rpm down the highway, I will probably do something different. Also, if Mercedes never built a stock 240D om616 with a 5-spd, Euro or not, to me it would be a good idea not to try it. Like I said Mercedes knew what they were doing.

Someone suggested a diff swap, and I've looked into that.
The car has the 3.69, and 26 inch tires.

3.69 @65 - 3100 rpm
3.07 @65 - 2579 rpm

So if the car wont pull the 5-spd, it wouldn't pull a 3.07 diff in 4th gear. Right? Itd be the same engine speed at the same road speed, now you've only ruined your acceleration in 1st,2nd,3rd, and you'll be wearing out the clutch faster.

I guess the only other option is to get a 3.46 from a 300 n/a or a 3.58 from a gasser. Heres the respective rpms, same parameters;

2906 rpm
3007 rpm

Neither of these give me the drop in rpm that I'm looking for, so I guess there's really no way to get a 616 down to those low rpm's and it still be driveable.

I will just find a 617 turbo, which is what I want anyway (who doesn't ) with its 3.07 diff, and pair it to my 4-spd. Sorry for the small book. Just my thoughts on it.

BTW, 26 inch tires from the standard 24 gave me a drop in rpm of about 250, and makes the car look a little better in my opinion. It also corrected my speedo!