I'd like to consider reducing the revs in my non turbo 300TD. Would a 3.46 to 2.88 be too extreme? I looked in the archives but I didn't find anyone trying this exact swap. Right now I'm getting about 25 mpg on the highway and don't want to kill the fuel mileage by over taxing the motor.

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Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)

What would you use on a W123 for a speedometer to maintain speedo accuracy?

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I would also like to know about this for my 1984 123 300D turbo
I hate running at 3500 RPMs on the highway

3,500 damn...how fast do you drive? Mine doesn't see 3,500 at 80 mph, not that I noticed anyway.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Me and my dad had the 83 up at around 3500ish rpm once when we were letting it out, but that rpm brought us slightly above 90mph!!!

If you are going 3,500rpm on the freeway I hope you live in Europe, or if here, can afford huge tickets.... I always drive around 73-75mph....which is like 2700rpm.....

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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)

You mean parts of germany.........Europe has speed limits and prety draconian fines.....and they love photo Radar.............they just mail you the fine.....

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

FWIW this is a 3.07 rear end - 3350rpm @ 75mph (verified by GPS)

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Never a dull moment at Berry Hill Farm.

That’s how fast I drive
The old girl has gotten used to it now but when i first got her she was dragging and screaming the whole way

I converted my '79 NA 300TD to a turbo, it was like a rocket ship with the 3.46 rear end, I could chirp tires even in the non turbo version especially if the pavement was wet! So I put a 3.07 in it. That was a fine balance between acceleration and top end.
Next vehicle, my wife's '83 300D turbo - I put a 2.88 in it and now it is one good cruiser. I do feel the drop in acceleration (its not had the ALDA tweaked)
Unfortunately now I am comparing everything to my '87 300D turbo (OM603). Ha! (there is no comparison!)
My advise is get rid of the 3.46 and you will find the 3.07 a great worthwhile mod provided your NA car has an automatic trans (if you have a stick shift then everything would be another story of course!!!
OTOH I would advise strongly against putting a 2.88 in a NA W123, you will suffer too much from loss of acceleration and the trans will downshift excessively or you will lug the engine unless you are primarily a long haul highway driver with no heavy loads (one passenger) and take no steep inlines (like Kansas?)

That said, be aware that a differential mod in the taller direction will increase fuel mileage which is (or will soon) become more important as we head towards higher oil prices Just be moderate and don't go all the way unless you have a turbo!
And don't worry about the speedo, they can be recalibrated, its the performance you want to go for in the first effort, worry about the readout later.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!