I have (2) 87 300sdl's
(1) Car has 161,000mi and at 55mph runs @ 2,5000 RPM and gets 23mpg.
(2) Car has 210,000mi and at 62mph runs @ 2,5000 RPM and gets 27mpg.

Is there any adjustment to car (1) that can be made to lower the RPM's
Injection pump timing?

That doesn't seem right....either your speedo isn't accurate on one of them or there is some major slipping going on in the torque converter....does the one with higher rpm at lower speed drive/shift any differently? That massive mpg difference also makes me wonder about the tranny.....it must be slipping....also could be an inaccuracy in the tach's on one or both cars....but the mpg thing makes me think the torque converter is having probs....something is slipping or causing drag....

__________________
-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)

I agree. Something seems amiss with the first vehicle. 2,500 rpm's is too high for 55 mph.

It would be nice to rule out a differential that is the wrong ratio, however.

You can jack up one wheel and turn it 10 times. Make a mark on the drive shaft and observe it while turning the wheel and count the revolutions. The drive shaft should make 58 revolutions if the vehicle has the 2.88 axle. It might turn more than this. Report back.

The car with 161k shifts perfect.
The only problem on car (2) with 210k is the hesitation of 3-4 seconds
when shifting to reverse. I drove it 150mi yesterday and it ran great.

sounds about right for a 2.65 vs 2.88 diff difference. so do brians thing and check it, i think.

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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.