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  #1  
Old 08-10-2007, 06:42 AM
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Regearing a 300SD differential

Things are going along pretty well fixing Guderian, my 1983 300SD. I'm getting close to daily local driving with it.

The next problem I foresee is the top end speed. 75 MPH was about 4000 rpm. Around here, 75 MPG might admit you to the granny lane.

Are there any gear swaps that might lower the rear end ratio? Where are the parts available?

If only there was an easy way to put in a manual transmission.

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  #2  
Old 08-10-2007, 07:07 AM
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With the 3.07 ratio.....75mph should only be about 3200 rpm. 4000 should put you more around 90-95mph
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  #3  
Old 08-10-2007, 07:39 AM
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You can put a 2.88 in there but with that large of a body you will need to make sure its quite spunky off the line.
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  #4  
Old 08-10-2007, 07:49 AM
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You can put in a 247 ratio from the v8 eaquipped cars. several have done it. I believe it is a bolt in swap.

Your takeoff from a stop will be a bit sluggish but once rolling you should be fine.

YOu will get better mileage too.

Tom W
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  #5  
Old 08-10-2007, 08:16 AM
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If you are looking for a fast car, you made a mistake buying the 300SD. You should have bought a 500SE.

You can drop in the 2.47 from a gasser W126 to lower the engine rpm's at speed, but it will take you longer to get to 75. The 2.88 might be a better way to go.

The fuel mileage increase from going to a 2.47 is, in my humblest of opinions, not worth the effort.
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  #6  
Old 08-10-2007, 09:31 AM
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Sounds to me like you've got a slippage problem, or a p/o put in an incorrect diff replacement.
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Old 08-10-2007, 11:18 AM
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I agree with PawoSD & Dieseldoofus, your missing 4th gear. Check your tranny fluid level as these cars are pretty finicky about that. Flat surface, running in Park, and warm fluid..........you probably already know the drill.
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  #8  
Old 08-10-2007, 12:14 PM
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Check and make sure that the car is really in D and not 3, as it could be a linkage problem.
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  #9  
Old 08-10-2007, 12:30 PM
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correct. if you look at the marks on your speedometer, you will notice the III mark around 75, that is the MAX rpm for 3rd gear. it should be shifting out of third into 4th at this point, unless your foot is on the floor... making the downshift switch preventing shifting out of 3rd. you could also have a bad kick down switch... a bad tranny, a bad shift linkage bushing, etc... fiddle with the shifter while you are around 65mph pop it into N and gently back into D and see what happens.
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  #10  
Old 08-10-2007, 12:31 PM
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Check the bowden cable as well.
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  #11  
Old 08-10-2007, 01:38 PM
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Run 215/75-14 tires. They fit, and will get you a 7% rpm reduction. And you will need new tires sooner or later, so your gear swap will be "free"
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  #12  
Old 08-10-2007, 02:18 PM
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3.07 versus 2.47

Just for fun, the attached graph represents the fuel mileage for each tank of gas I bought for my 1984 300SD before I sold it. Can you guess what date I changed the 3.07 for the 2.47?

P.S. - I did not drive that car in the snow, so that's why the December to March period is drawn out.
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Regearing a 300SD differential-mileage.jpg  

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