Cummins mpg vs 225 Slant 6 mpg

If it's a factory Cummins and not a conversion go the cummins. The 1st. generation can get closer to 25 MPG. They however will not go as fast MPH as the newer Diesels because the gearing back then was different and you did not have the overdrive ratios they have now.

Does take more$$$ to change the oil & filters versus the slanty.