My 89 Cummins (std cab, 4WD, 5 speed Getrag) will consistently get the following:

Highway, unloaded: 21-23mpg.

Towing a car trailer with car: 17-18MPG.

Towing a 27' Airstream: 14-15MPG

Commuting: 20-21MPG

Numbers have stayed consistent over the last 13 years.
Fuel pressure has been cranked a bit, but truck is basically stock and doesn't smoke at all. I also tend to drive for mileage, not for speed, that's what the 560SEL is for....

Jim

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