With a stock size tire and stock 3.92:1 diff I think, you should be turning around 3200rpm @ 60mph. Autos a bit more as the converter slips, they are non lockup units in these cars.

Remember, the car has 62hp I think... maybe it was 67hp other years like my old one. It is made to dork around town, not for top speed runs on the open road. Would I drive it at 70mph? Sure, will not hurt a thing, 80? Eh, maybe some. Not a chance in hell I would swap to a taller diff than what is factory installed. Not enough power to move with the A/C running.

My 300SEL has a 3.46:1 and I think this would work fine with a 2.88:1 and first gear starting. But eh, I am accustomed to it turning 3400rpm @ 70mph and around 5200rpm @ 110mph. Motor is fine with this lifestyle.



Lash those valves, have the injectors redone, and enjoy the roads. :)

Wow guys I really didn't expect so many of you to reply haha. Thanks again for the help. Of course I didn't know much going into this (and I've still got A LOT of learning) but know i know a little more. I'm sure I will be back with more questions. Thanks again

That's what we're here for man! Glad that we're able to help you out. I've also attached our site's DIY tech articles for your W123 - use it to troubleshoot and resolve issue. If you have further questions, post a Comment just below one of the articles and someone will get back to you with a more detailed answer. Best of luck!

Mercedes-Benz W123 300TD (1977 - 1985) Technical Article Directory | DIY Maintenance and Repair Projects for MBZ 230, 240D, 280 CE, 300D, 300TD | Pelican Parts


-Dmitry

As fast as you can if the car is in good mechanical shape!

These cars (even the underpowered old Diesels) are built for Autobahn cruising at high speeds and high revs. As long as your mechanicals are in good order, then zoom away :)

Only if the speed limit allows it ;) And if you have a long enough stretch of highway to get the car up to that speed :) Just joking!

With the non-turbo diesels and even the turbo, the thing to watch out for, is the lack of acceleration compared with a gas engined car. You need space and time when entering a fast moving stream of traffic.

We at one time had a little 3-cyl diesel smart car. Great little car, but didn't like to go much over 60mph. Had to drive in slow lane of highway and contend with the trucks. You may find the same thing. But your car should be fine at those speeds.