5.9 Magnum into a 73Duster

Mark Nixon said

If you are looking at the V-8 Magnum for excellent mileage, forget it, the very best I have seen is @ 18, not surprisingly, it was a non-over drive automatic 318 in a '92 Dakota, 3.21 gears, with a hollow cat.
Overdrive, even manual, really seems to affect the mileage on them.

A TBI or carb does better, mileage wise.

Mark.

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I routinely get 18 out of my 1997 Ram. 5.2 5 speed, club cab, short bed, 2wd. 3.55 gears. Tonneau cover Lower 20's is possible if driven conservatively.

I have headers, Magnaflow converter (stock configuration) and Magnaflow exhaust. Parts store tune up stuff, with Taylor wires.

My MPG is directly tied to MPH. Anything between 58 and 64 gets best MPG. Under 58 in OD is too low RPM and engine lugs. Mileage drops over 65, purely because of RPM. The OD in the 5 speed must not be very deep.