gotcha... so how tall of a tire has been fit under stock wells? i have the XHD springs right now and a want to run 3.73 or 3.91's. so i need like a 28 or so to run around 3100 @ 70?
27-1/2" is about as tall as you can fit without the front of the tire contacting the wheel opening. Springs don't affect that allot. I have the XHD springs on my 68 Cuda and had 28" tires on it and there was hardly 1/2" between the front of the wheel opening and the tire. I put 27" on it and it clears fine. If your running an automatic it'll slip a little so you have to figure that in the equation. I have a 3.73 gear in mine and with the 27" tires it spins 2900 at 60. It's been awhile since I took it up to 70 but I think that was about 3400 rpm IIRC.
