I used a 2x4 (laid with 4" side down) and a square to figure out how much roll out my tires had with X amount of air. I started at 25 psi and went up to 35 psi, in 5 lb increments. There was a bigger difference between 25-30 than 30-35. I forget the exact measurements but there was 3/32" difference with 5 lbs of air change. Also a "race" tire may change differently than a "street" tire.