Noob rear diff question - Page 2 - PeachParts Mercedes-Benz Forum

BATComm1966 · #16 12-20-2018, 08:45 PM

Dang

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Stretch · #17 12-21-2018, 06:54 AM

3.07 with an OM617a (turbo) would be just fine - much better than the original differential that was intended for the 240D

But much like your car - parts get changed - a more safe way of finding out the final drive ratio is to crawl under the car and look at the lower left hand corner of the casing next to the differential's rear cover (so that's the left when you are looking from the rear towards the differential). You might need to clean off gunk and rust to see the stamped numbers so take some oil and a bit of scotch brite with you (same could be said for your existing differential which might have been changed with the engine - this could save you buying what you already have!)

BATComm1966 · #18 12-21-2018, 11:02 AM

Thank you Strech, I've done just that on my 240d, 3.69 ratio the other car is in a small salvage yard that I know the owner of and the car will have to be lifted from where it sits to get under it. Still has wheels and such on it just has sunk enough to bar access.

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BATComm1966 · #19 12-21-2018, 11:07 AM

I've been wondering about the 3.07 ratio, which I already have that the PO gave me when I bought the car.. I'm going to be doing allot of highway/turnpike driving and will need the 75/80mph capability, with the 300TD transplant and the 4spd trans.

Getting consistent answers on this ratio are hard to get but the 2.88 mention gets me all positive "yes do that" answers

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Stretch · #20 12-21-2018, 12:03 PM

Quote:

Originally Posted by BATComm1966

I've been wondering about the 3.07 ratio, which I already have that the PO gave me when I bought the car.. I'm going to be doing allot of highway/turnpike driving and will need the 75/80mph capability, with the 300TD transplant and the 4spd trans.

Getting consistent answers on this ratio are hard to get but the 2.88 mention gets me all positive "yes do that" answers

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There are plenty of calculators on the interweb that will give you a good idea but it is just simple high school mathematics

Work out the circumference of the tyre - final drive ratio in top gear (1:1 on a four speed) and see the theoretical speeds that can be achieved at say 4000 and 5000 rpm

But seeing as you've got the 3.07 => What are you waiting for?

If you found that option lacking then you could hunt for a 2.88 or even as a side step fit larger tyres on the back (I can't remember what "they" say will fit - but that's also got to be found quite easily on the interweb)

BATComm1966 · #21 12-21-2018, 12:07 PM

Very true on the 3.07, just wanted to do the job once, since this will involve changing the speedo cluster of the gears in the cluster to keep everything correct.. Symantics I guess..

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Stretch · #22 12-21-2018, 12:09 PM

So long as the speedo doesn't have to be true for safety checks (I dunno about where you are but here in Holland they don't really care too much even though it messes up the odometer readings) then you can use navigation / mobile telephones to help stop picking up speeding tickets

Dan Stokes · #23 12-21-2018, 12:25 PM

Quote:

Originally Posted by BATComm1966

Very true on the 3.07, just wanted to do the job once, since this will involve changing the speedo cluster of the gears in the cluster to keep everything correct.. Symantics I guess..

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I got nothin' on the Mercedes final drive ratios but I can tell you my experience with my OM617 in Mutt the Race Truck with the GM 3:08 and a 240 Getrag 4 speed manual. (Remember, the engine doesn't know or care where the final drive comes from, it just knows the ratios).

Anyhow - the 3:08 seems about perfect. It can pull off the line just fine (not adequate for a drag strip but fine for Land Speed Racing and it seems like it would be OK for daily driving). I shift at about 5200RPM (not a misprint) and cross the line at about 4800 or so (I'm pretty busy at the top of the track and don't have a lot of time to check the gages) for a top speed around 136. This is with a 28" tall rear tire.

Of course, a higher (numerically) gear will give more take-off power but at the expense of top end. I don't know what tire diameter you run but with somewhere around a 28" tire I'd go with the 3:07.

Dan

BATComm1966 · #24 12-21-2018, 01:00 PM

Speedo doesn't have yo be on here just me wanting it to be correct, Dan it just my 240d with stock size tires I'll have to check the diameter.. Been searching and starting to get a handle on this, the 3.07 may work...

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BATComm1966 · #25 12-21-2018, 02:07 PM

Found a couple calculators that hit about the same with the different ratios, looks like with the 3.07 and the tires I have now ( 24" diameter) at 80mph should be around 3500 RPM, got to the 2.88 and I drop to around 3250 RPM, not as big of a difference as I was thinking... Guessing that those sustained RPM's wouldn't be an issue with the 300TD... Guess I'll be going that route!!

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