It all started, doctor, when I was in the garage waiting for my wife to come out. There was a tape measure lying out, so on a whim I decided to measure the diameter of my tires. And they measured ... drum roll ... 29 inches. WTF? So I measured again more carefully and decided it was more like 29 1/4.
The nominal diameter of those tires when they were new was 30.5" with a tread depth of 16/32 = 13 mm. That's probably on a 7" rim and I have an 8" rim.
The tires don't measure the same horizontally as they do vertically, and the wear isn't completely even. But the diameter is about 29 1/4" and the axle center sits around 14 1/2" off the ground.
The tread depth of the more worn tires is about 10/32" = 8 mm. That's a wear loss of 6/32, which would be a 6/16 = 3/8" change of diameter. That would explain a 30" diameter, but not 29 1/4".
Am I missing something? I checked and double checked the sidewall labels. No surprises there. Surely 8" rims don't make that much of a difference?
