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      02-05-2013, 07:41 AM   #6
b1aze
Just one more taste...
United_States
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Drives: 2010 135i
Join Date: Aug 2012
Location: Shirley, MA

iTrader: (14)

18" wheels fit a tire with a specific INNER diameter (18") however the OUTER diameter can vary based on the tire width (the first number in the tire size specification eg. 245) and the tires aspect ratio (the second number in the size eg. 40). The aspect ratio is a % of the tire width which equals the sidewall height. If you take 245 * .4 = 98mm * .0395(mm to inch conversion) = 3.871 * 2(because you have to account for 2 sidewall heights in a diameter) = 7.742 + 18(base diameter of the wheel) = 25.742" rolling diameter. Deviate from this diameter too far and you will have an inaccurate speedo and some people claim potential brake system failure.