Not having my abacus on hand, I used an online calculator (
http://www.advanced-ev.com/Calculators/TireSize/) to calculate the following:
Both trucks traveling at convoy speed of 25 mph:
F30 with 7.16 diff ratio and 37" diameter (10.50-16) tyre: engine runs at 1626 rpm
F60L/FGT with 7.16 diff ratio and 41" diameter (10.50-20) tyre: engine runs at 1467 rpm
Pretty much the same? A heavier (loaded) truck would of course use more fuel.
Still some homework to be done. Grant, does the AEDB list 9.00-16 tyre diameters so we can compute F15 and F15A engine rpm @ 25 mph?