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*To*: bbell@csn.net (Bruce Bell)*Subject*: Re: Snow tire size for '86 4kq*From*: shields@tembel.org (Michael Shields)*Date*: Sat, 26 Oct 1996 03:49:37 +0000 (GMT)*Cc*: shields@tembel.org, dans@ans.net, sabutin@mindspring.com, quattro@coimbra.ans.net*In-Reply-To*: <199610251819.AA09780@ns-1.csn.net> from "Bruce Bell" at 1996-10-25 12:14:20*Sender*: owner-quattro@coimbra.ans.net

> Careful. Did you compute revs/mile from the tire size? Won't > work. there is something called a static radius when there is > weight on the tire. Typical revs/mile for a 195/60 14 is around > 916 (D60 A2). and the Dunlop Graspics are closest to this in the > 175/65 14 - according to the Dunlop full line book. > > There are substantial differences in tire Models and > manufacturers. I just gave the output of the tire_size utility, which does the calculations in the obvious (naïve?) way, by calculating the sidewall height based on width times aspect ratio, adding the diameter of the rim, and multiplying by pi. If I understand what you're saying, though, this uses the unloaded sidewall height, and the tire will have a different circumference under load, the difference depending on its design. Since the D60's 916 revs/mile differs from the calculated value by a full 5%, this means that you cannot freely switch between two tires, even of the *same* nominal size, without knowing how much their sidewall height will change under the mean load you intend to put on them (and what is specified is probably the deformation under maximum load). Is that right? If so, why don't tire vendors tell you this? -- Shields.

**References**:**Re: Snow tire size for '86 4kq***From:*"Bruce Bell" <bbell@csn.net>

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