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75% RULE I used to call my soft-going tire pressure guideline the 75% rule, but that needs a modern tune-up. My rule is perfect for 15-inch wheels, but needs adjustment for 16-, 17-, 18-inch and larger wheels. The newer, larger wheels and lower "profile" tires complicate my old rule. For example, on a P235/65 R16 size tire, the "65" (percent aspect ratio) is the "profile." I have not completely run larger wheel, lower profile tire sizes through my analytical ringer. For these, I'll make an educated guess.
Assuming you're not running 65 or less profile tires, and you are using something in the range of 70 plus tires, consider this estimate. I'd reduce the 75% rule by 5% for each inch of wheel increase beyond 15-inches. For example, a 16-inch wheel should use 70%. Now follow along with the figures and throw your percentage in when I use 75%. |
