Gas powered Mercedes that I have been in have driven comfortably at 90 to 100MPH. At 85 MPH, my 300SD is well over 4000 RPM and sounds like it is ripping its guts out. Are there replacement gears available to change the rear end ratio? Are there diferentials from other models that can be easily substituted?

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1983 300SD "Guderian"
1987 MR2
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It should not be revving that high at 85 unless you have just run the speedometer out past the 85 mph stop. What is it revving at 55? Mine runs at around 2500 at that speed and around 3500 at 80~85. Is your transmission slipping?

Yo can replace the rear end with one from a 420SEL. That has a ratio of 2.47:1 which is higher than the 3.07:1 in the SD. I bought one last year, but have not put it in yet. Do a search on differential swap and you can see some threads on the subject. You will also need to either modify the speedometer or get a new one once you change the rear end ratio.

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count the shifts. I bet you are not getting out of 3rd.

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4000+rpm at 85mph sounds right with the 3.08.. if you are looking for 85-90, put in the 2.47 and you will be all grins.

Nope, with a 3.07 ratio and stock tires it should be about 3550 rpm at 80 mph and 3750 rpm at 85 mph. If you are really over 4000 rpm, you may have a slipping transmission.

Or The Torque Converter Is Going Out?

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

You sure about that? My 300TD was 3000rpm at 65mph with a 3.07.

4000rpm at 80mph is about right.

Sorry, I was off by 500 rpm, make that about 3550 at 80 mph and 3750 at 85 mph. I still cant see it being over 4000 without the transmission slipping.

I'll edit the first post.

If you were doing 3000 rpm at 65 mph, you would be at about 3700 at 80 mph, that sounds about right.

Mine is about 3550 at 80 mph, but I do have 205 tires and my transmission and torque converter is pretty new.