ymmv.

Has a lot to do with the engine tuning. Cam chain enlongation on these old 617s have shifted the torque to an even higher rpm now. Perhaps if if seems like you could use another gear while running down the hwy at 65-70 then maybe... However, if it takes a fair amount of throttle to run at that speed now then I'd say it will take more fuel with the lower numerical rear end.
Although, I'd say for a fact that my 1 ton cummins powered truck would see an increase in MPG. The SD... I don't think so. The TD, definately not, I'll stick with the 3.07.
I think MB had it right off the ass'y line. I don't think we can do much better.

So you are saying that your imagination trumps my real life experience with changing the diff ratio in my benz?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Then you better get a 2.88 in your 300SD's.

what w126 diffs have people put into turbo w123's with what result? what would be the best w126 diff to increase my low-end power on the w123 turbo?

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If you want more acceleration you need a higher number in the gear ratio. Your car is a 300d turbo? If you want more acceleration and your car currently has a 307 diff you would want perhaps a 346. You would gain acceleration but lose economy.

If you wanted better economy you would want to go the other way, perhaps to the 247 diff.

The only way to have both is to get more gears to work with in your tranny.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

so at what speed and rpm range would i stop feeling the acceleration benefits of a 3:46 diff in a w123 turbo? over a 3:07, that is?

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Well at cruising speed you would have lot more engine noise since it would be turning 20% more rpm on the highway. You'd use about 20% more fuel too....in round numbers.

But the car would feel more peppy at all speeds up til it ran out of rpm.

The top speed would be limited by the governor to about 90-95....instead of over 100.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

The first pic in this thread shows the type of wear I am seeing. I'm pretty sure this type of wear classifies it as being "shot".

Excessive W123 Differential Wear - Pictures


This creates a "heavy/dramatic" clunk when putting the car in gear and when down shifting at slower speeds.

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AJ

1985 300D (SOLD)

Uh... yea.

Do this Tom, next tank of fuel change your driving habits. Start off in 2nd gear, shift to 3rd quickly before the engine nears its peak torque. Do the same 3-4, 4-5th. In addition to the crankcase oil temp rising I'm betting your fuel mileage drops.

I always got better mileage from my 83 SD with it's 3.07 than I do with the 85with the factory 2.88. The 2.88 is fine in the diesel W126, I just wouldn't want a 2.47 for my drive.

But, for those who want to go to the trouble of swapping ratios, speedometers... go ahead. Don't let me stop you. I'll gladly sell you a differential.

I have a 2.47 out of a gasser in my coupe. I regularly get 31-33MPG in mixed traffic. speedo is off by 40% which make the cruise control function interesting, but otherwise I do the speed calculation in my head. one of these days Im going to pull apart a 300 speedo and see if the gears can be changed to get a better reading, but it doesnt bother me and gives me a smile at the pump.

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)

how about a 3:58 diff out of a 230e w123, into a turbo w123--would that achieve about the same results as a 3:46? which is preferable if low-end power is desired--the 3:46 out of the diesel or 3:58 out of the gasser?

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The difference between the two is negligible.

If you REALLY want low end power, get a 2.92 out of a 200D or 220D.

so, it's not correct that if i put a higher ratio diff that I get better low-end power?

what would the 200d/220d 2.92 limit the top speed of a w123 turbo to?

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The larger the number the greater the leverage the engine will have against the wheels.

80mph will be the top speed at 4500rpm.