I have read in several places, one of which is Muscle Mustangs and Fast Fords, that you want your rim width to be as close to the tread width as possible. A 12.50 tire has a tread width close to 10 inches. My 33x1250x15 Thornbirds are 9.8 inches wide, and should thus be used on 15x10 wheels. However, I run them on 15x8 black mods. The other advantage is that the 15x8's are cheaper thatn the 15x10's, and that they do not stick as far outside of the fender as the 15x8's. I haven't had any problems with the tires on the 15x8's, except that the place I took it to to get an alignment said that their machine could not reach around my tires to grab the rim, and so they could not do the alignment because the rims were too narrow for the tires. I took the fron tires off and mounted my mom's mustang's front tires and drove it down there (looked pretty funny, I should have taken pictures) and was able to get the alignment that way.