IIRC my '91 runs around 2700 rpm on the highway. I'll check this morning on the way to work and post back.

I commute 70 miles/day running at 60-65 mph (state roads, not freeways) and I'm getting 32+ mpg. Best tank so far was 34. Worst was 22, but that was prior to replacing the fuel filters... I'd replace those first.

Other than keeping your eyes open for the potential issue sixto mentioned, I don't think the w124/602 diesels have any endemic problems. I had to replace a flex disc, thermostat and the serpentine belt tensioner/arm/spring/pulley within the first two weeks I owned it.

The fuel warmer on the passenger side of the engine seems to be a common problem. I bypassed it (after it dumped a gallon or two of diesel in my driveway).

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1991 300D 2.5 "Rocinante"
2002 Golf TDI "Teen Spirit"
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1984 300D
1966 Mustang I6 3sp
1985 Mazda RX-7 GSLSE
1982 Toyota Supra
1977 Datsun 280z
1971 Datsun 240z