Type Three Tuning Page -- Tires
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If the aspect ratio of the tire is not specified on the sidewall it is 85. Thus
a P165R15 would normally be designated a 165/85R15. 85 is the default aspect
ratio for P-metric tires so they normally don't put it on the sidewall.
Euro-metric tires have a default ratio of 82 (but it is being phased over to
80). Thus, a 165R15 is actually 165/82R15.