Linus Gump
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It all depends on what the rest of the numbers are. 315 is the section width of the tire in millimeters, (the total width minus any raised lettering). The next number over, say, 75, is the aspect ratio. This is the percentage of the section width that the side wall is tall. So using the numbers we already have, 315mm is approx 12.86 inches wide, and 75% of that is 9.6 inches. This is from the wheel to the tread, so we have to multiply that by two and add the wheel diameter to get the overall diameter. With a 17 inch rim, that works out to be 2(9.6)+17=36.2 inches tall. On a 16 inch rim, it is 35.2 inches. On a 20 inch rim it is 39.2 inches, although I doubt anyone makes one that size. If the aspect ratio drops to, say, 50, then on a 17 inch rim it is 29.86 inches tall.
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tire question - 
