How fast should I drive my 240D?
 

Hello all,

I apologize if I'm not in the correct thread location.

I have recently bought my first 240D. It's also my first diesel car. And my first Mercedes... So basically, I'm a complete noob on this topic.

My question is, is ok to drive 60-65 on the highway? I just feeling like I'm revving the engine to the moon at such speeds. The car is an 83' with the 4spd manual. I'm assuming it has the higher ratio rear end, and that at 60 mph the engines turning at about 3000 rip 'ems. Could I swap the differential for say the 2.88 from another car? Would that even be beneficial? Where would I get said diff? I live in a flat area so hills aren't a concern. And hey, I bought a car with 64 horsepower, I'm not a speed demon either.

Thanks for the help.

First, congratulations on your 240D! No problem with high rpm. My '88 300SEL also runs at the same high rpm. Most 4 cylinder diesel pickups in the rest of the world also runs the higher rpm without problems.

As far as swapping the differential, I will leave that to the experts!

Assumptions are dangerous. Inspect the differential to verify which one you have.

I would not change the diff
 

I think you would be sorry you did. Flat terrain or not the car would not be able to accelerate acceptably or possibly even safely. Benzo engineers know their stuff and selected the ratios for the best performance - economy tradeoff.

I have been advised by MB that the 616.912 engine + MT should not be driven for sustained periods over 4500 rpm.


I have driven at 3500-4000 at 70-72 mph for extended periods with no ill effects.


This car has 132,000 miles and is kept in top shape valve- and injector-wise.

What it all really depends on is the cruising rpm for the vehicle engine. That's the one for which high-speed cruising (Autobahn) is designed.

FWIW

Thanks guys for all the inputs. It makes me feel a lot better knowing that I'm not ruining my brand new (to me) Mercedes by driving her at 60-65. By the way, it has 266,000 miles. The engine is in fantastic shape and was well taken care of. I suppose I'll stick to regular maintenance and see how far she goes. Thanks again for the help

I have driven mine at 70 for years with no ill effects.

Most likely the car will outlast you. I am still racing my 1981 300SD to see who gets to the end of the trail first. So far it is doing better than I am. Good luck. They do not make them like this any more.

IIRC older (late fifties/early sixties) Mercedes owner's manuals would state a cruising speed and a top speed. Often they were the same.

knightwrider, it strikes me that any engine speed below the Red Line on the tachometer should be OK indefinitely. As mentioned, MB diesel engines last for hundreds of thousands of miles if maintained properly. It's likely the body will suffer from major rust issues before the engine dies.

As mentioned by other members, older diesel engines tend to run on high RPMs with zero issues - maintenance is key. Enjoy the new ride.


-Dmitry

I ran my OM615 manual up to its top speed of 84.3 mph. Not for overly long periods, but she was able to get there and stay there for at least a few miles at a time. I found that she was most happy cruising at 70mph on trips to Portland (350 round trip). Mountain passes seemed to top out at 60mph on steep slopes. Speed drop from 70 to 60 over a five or six mile climb

You can do as fast as the car will allow, but I wouldn't do that for long or often. I was able to get my 77 240D auto up to 95mph but usually it did fine going about 75mph, anything above that and the temp would start to creep up, not sure if the cooling system was in top shape though.

With a diesel, the governor in the injection pump will limit RPM by pulling back the fuel. Have a look at the numbers on the pump / governor. Many times the RPM is listed in the number but remember, this is pump speed so double the value.

I though that, with all Mercedes, you drive them as fast as they will go. I used to do that and the others on the Autobahn seemed to be doing likewise. 80 million Germans can't all be wrong!!!

RayH

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.