>Measure by = wrapping a string around the outside of the tire...>

>Ahh... but don't = forget you have to do that with the tire ON the car...>

Thanks Bill and = Brent. Is this a Friday thing? Or maybe it's some new list member = initiation rite? :)

Seriously... = assuming the tires you buy will be of uniform dimensions if they are = the same brand, model and size, you can figure all this out without = driving on strings, etc. After all, unless we're doing speedo = calculations, it isn't the actual dimensions we care about - only the = differences - right? So your average, brand new 195/75R14 tire has a = diameter of 648 mm, a circumference of 2036 mm and revolves 790 times = per mile. If it doesn't, it'll at least be close enough that the = difference is irrelevant. Every mm of diameter +/- accounts for +/- = 3.1417(pi, you know) mm of circumference. Each +/- 2.58 mm of = circumference accounts for approx. +/- 1 revolution per mile. = "Approx." because as the circumference increases, the mm to = generate another revolution increases as well - and = vice-versa.

So it seems to me = that if my tires are approx. 648 mm in diameter, a difference in tread = depth (sampled from various points on each tire, of course) of 1 mm x 2 = (for each side of the tire) would give me tires with diameters of 648 = and 646 mm respectively. Which would yield circumferences of 2036 and = 2030 respectively. Which would mean that the tires were rotating 790 = and 793 times per mile respectively.

Now... if I had to = take into account tire load and its influence on revs per mile *so = would someone with brand new tires* - so I don't see the relevance in = considering it. I'm figuring - and I'll admit perhaps incorrectly - = that the difference in recommended tire pressure front to rear most = compensate for whatever effects load might have on the = tires.

So will a difference = of 3 revolutions per mile toast my VC? I sure don't know. Anyone who = does - please let me - and everyone else - know!