As all drivers of w123 diesels have noticed, these engines don't sound that great at freeway speeds. Aside from lowering final drive ratio (making this car even slower in city driving), or swapping in a 5-speed (extremely hard to find), it seems that an aftermarket electrical overdrive may be best bet to lower freeway rpms. My dad's '81 volvo 240 has a 4-speed w/ electrical overdrive and it seems to work as well as a regular 5-speed.

Anyway, I was wondering if it is possible to add an aftermarket overdrive onto a 1980 240D 4-speed manual, and if it is worth the effort. Any comments/opinions are appreciated.