I have a 55.1 with a 3.9 rear end. I switched from a 3 speed to a 4 speed with a granny low. I took off the non-original 15" tires on the rear, and put on 16" bias tires with a 31" diameter. It made 13% difference in the speed from my old tires. You can figure yours by figuring the diameter ratios. I am going to a 33 on the back and put the 31s on the front. Then I will be going down hill all the time...think of the gas I'll save! (I know, I know) - Scott
