I have a set of 255-60R-15 Mickey Thompson ET street tires mounted on a 15x8 rim with a 4-1/4" backspace. I also have the MP SS springs. There is a 1/4 inch clearance from the inside of the tire to the leaf spring and about 1/2 inch of clearance to the fender. Air down the tires to18lbs and you will get good traction. I too have 3.91 sure grip. And a 340 that dyno'd at 410hp. Hope this helps.
