Have you checked the circumference of both tires? even a 1/2 to 3/4 inch difference could cause that also. I just got a set of slicks in they had 109 1/4" and 109 1/2" writen on them. When I mounted them one was 104 1/4" and the other was 103". I aired the 103 to 30 psi and set it in the sun for a while. Remeasured it measured 104 1/2". I'll keep checkin since I was told it will gradually try to shrink back.