Somewhere in my foggy memory I think I read that the 300 SD has a different differential than the 300D. Hence better highway performance and milage. Then just recently it was suggested that they have identical drive trains. Since my new (to me) SD has a non operative tach, I cannot compare. I'm sure there are plenty of folks who could educate me on this point. I'd like to know, so any input is appreciated.

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John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln

In 85, both the SD and D had a 2.88 rear diff. Earlier than that both had 3.07 rear diff. You are probably confused by the years rather than SD to D.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

That's what I needed to know. Many thanks

__________________
John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln

If the drivetrain is the same on each I would think that the model that weighs less would have better performance.
Also compare the diameter of the wheels of the 2 models.

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84 300D, 82 Volvo 244Gl Diesel

I thought the same thing but the EPA states the same mileage for both the 300D and the 300SD.
http://www.fueleconomy.gov/feg/findacar.htm

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

It was a long time ago but when I saw them doing an EPA test on TV the CAR was on something like a Dynomometer. No on the road testing. The reason they say "Actual Mileage May Vary".
So, I believe during the EPA test a slightly heavier car could have the same EPA milage as the lighter one with the same drive train.
But, on the Road the slightly lighter weight one may accelerate faster handel a little easier and exhibit other differances that people look for when they use the term better performance.

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84 300D, 82 Volvo 244Gl Diesel

My 300D is a bit more peppy (peppier?). However the D has 215K and the odometer on the SD stopped at 310K. So that may cause some difference. I think the wheels are the same. At any rate, they both run and ride great -- I was just hoping for higher gearing on the SD. About 75 is a fast as I like to cruise in the D.

__________________
John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln

While the W123 (300D) is physically lighter, the 300SD (W126) is more aerodynamic, so they probably come close to even in fuel economy, and the W123 will feel slightly quicker. A younger engine helps too.

Both of your cars will have the 3.07 differential since they are both '83's

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)