Oh by the way, tire wear and under inflation does effect the measurement. When I was double checking this thing first measurement I got was 3/4 inch off from what the illistrator said. then it dawned on me got the air pessure gage and I was @ 20 lbs not the 30-32. So I went to 32 and what do you think? I was only off 1/4 of an inch.. I looked at the tire wear and I know where the other 1/4 inch should have been..