Actually, in the 1:1 world the front axle is a wee bit faster, it's pulling the tractor rather than holding it back. It's only noticeable when you turn it off.
Having same rpm motors with 25% difference in tire diameters will without a doubt be too extreme in different speeds & destroy the model quickly. Try finding two different gearheads with roughly comparable final outputs to the ground & if one is a little faster, be sure it's the front steerable axle.
