My stock 52 3100 got 15.0 MPG empty, and 15.0 MPG with a 3/4 ton pallet of potash in the bed.
You have something seriously wrong somewhere.
Tire diameter way smaller than stock?
Odometer?
Measurement of fuel used during your test is wrong?
Calculation error?
I am hoping its one of those. They are easier to "fix.
Carl

Last edited by 52Carl; 06/02/2015 12:58 AM.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission