Actually the this string 'measuring' = thing was only to compare wear on tires of same brand, size, model, etc. (I think = someone mentioned that their tires wear more on one side that the other.) And to = do that they don't have to be on the car as long as you measure all of them off = the car in the same manner at the same tire pressure. :)
Bill
-----Original = Message-----
From: Fitz-Randolph, Douglas <dfrandolph@TALKAM.NET>
To: vanagon@GERRY.VANAGON.COM <vanagon@GERRY.VANAGON.COM&g= t;
Date: Friday, April 16, 1999 2:32 PM
Subject: Re: Syncro = tire dia. spec. - tolerances? Ask Mr. Science!

>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!