Best gear ratio for 15 speed

Those years of d12’s weren’t very strong that’s why someone geared it that way.. I doubt if you pick up much for fuel mileage . It will take many years to break even , your best time to regear is when one rearend needs parts replaced..

 

If the rear ratio is in the 4’s then it is definitely an overdrive. A direct would be geared something under the 3’s.
I had a 15 over with 3.73 rears and a series 60. It worked pretty well.
The 15 over had a top gear ratio of 0.79 vs 0.73 for a 13 or 18 speed.

Joel Morrow said even the old D12 was a European engine designed to run really low rpms. With your lighter loads it would probably work well with something in the low 3’s for a rear ratio. Need to know tire sizes to get better numbers. Or maybe ask him on facebook.