one fairly easy way(if, you're handy with a wrench) is to drop in a rear end from a 1985 300D . or a wagon , etc:-
The 2.xx ratio will give you not only a higher cruise speed, but also a good 2 mpg gain. Plus, the car wont be so 'buzzy' at highway speeds

It really is a bloodless bolt-in, 1 hr at most.

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1982 300SD (214 k, excellent shape) kidmobile running mostly biodiesel, gets 31 mpg hwy with 2.82 rear from a euro 500SEL



1976 115 body 240D 4-speed (traded for Jeep parts) - Engine lives on in my CJ7