Jimberg, that is a good and easy formula. Is the 1270 a constant? Just wondering were that (1270) came from.
I usually do a rough figure, 225/25 x .5 x 2 + 15 = 24. The reason that is rough is that 25mm is not exactly an inch, its 24.??. Your formula looks to be exact and much easier.
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Rick
My 89lx(updated 2/21/2001)
My 97 Cobra