How do I get new rear end gear for my 1980 300sd?

[gberger]

My car is equipped with the 4 speed automatic transmission.  The rpms are way too high on the interstate/freeway roads.  I would like to reduce the rpms on the highway.

How do I get a new rear end gear or rear end assembly???

[Big_Richard]

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[TJ 450]

Just something to note, is that the V8s have a larger flex disc than the 280s. I'm not sure whether this applies to the SDs, but you would need to swap the flange on the diff if that's possible... Research will need to be performed.

Tim

[Big_Richard]

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[John Hubertz]

Here's what I would do:  First, determine what year or years and model of car generally came with the ratio you want to have.  Then, check for availability at http://www.car-part.com (largest online inventory of junkyard parts in the world).  You'll probably be pleasantly surprised at the prices you find - but whether there are any available anywhere close enough to make shipping practical is another question.

[jbrasile]

gberger,

Your car already has a 3.07:1 rear end ratio (which is the same as the 450SE/SEL's by the way) which should give you around 3000 rpm at 65mph correct? Installing a higher ratio diff  such as a 6.9's 2.65:1 will kill your off the line acceleration. Remember that even though the 300SD has very good torque characteristics, it is not a large displacement V8 and while the turbo kicks in very early in these cars, it cannot compensate for such a tall gear ratio. My opinion would be to just leave it alone.

Tks,

Joe

[ronw123w124]

How about a 2.88:1 rear end gear from a laet w126 300SD

[jbrasile]

Hmmmm....

Now that's an interesting thought.... the 85 300SD has basically the same hp and torque as the 116 so I suppose it might work.

Tks,

Joe

[Big_Richard]

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[jbrasile]

MT, you might be right.... looks like up to MY84 they are interchangeable.

EPC says P/n 116 350 83 20 goes upto 126's model year 84

85 onwards you have 126 350 63 03, however if the swap includes the drive shafts, it might be doable.

Tks,

Joe

[Big_Richard]

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[TJ 450]

The late model W126 diff housing is modified so that it is better insulated from the sub frame, and features additional mounts.

Tim

[ZCarFan]

I agree with jbrasile that changing the ratio will hurt initial acceleration.  The ideal solution is to use a transmission with overdrive.  I have seen conversion kits to mount  the GM TH700R4 (aka 4L60) to a OM617 though I am leery of the "custom" torque convertor that is required.  That would leave you with a steep first gear (3.07) and a high overdrive (.70).  If the transmission is built right by someone that actually knows the product, it should deliver 200K+ mile service.  It is more than strong enough for this application.

naturally, the amount of money spent would be very hard to recoup unless you drive a lot on 65-70MPH roads.  I spent some time looking around and thinking about the problem and the only solution I thought would provide any significant savings overall would be moving to a 5-speed manual.  That should provide huge benefits in acceleration along with fuel economy which would offset the costs.  I came to the conclusion it was just better to just use it as-is.