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Since this formula has been put in numerous posts but questions still pop up from time to time I thought I would throw the formula here.

Ex. tire size. 275/40/17

Breakdown
275 = width in millimeters
40 = aspect ratio of width (40%)
17 = wheel diameter

Note.. 1 inch equals 25.4 millimeters

Obviously to determine width in inches you would just divide 275 by 25.4
275/25.4= 10.83 inches

To determine overall diameter of wheel/tire combo..
40% of 275mm = X
275mm x .4 = 110mm

Note that 110mm is only one sidewall and must be multiplied times 2 since you have 2 sidewalls when looking at the overall diameter of a wheel/tire.
110mm x 2 = 220mm

At this point I convert the sidewalls to inches
220mm / 25.4 = 8.66 inches

Add your wheel size (17" in this example) to 8.66 inches
17 + 8.66 = 25.66

So a 275/40/17 is 10.83 inches wide and 25.66 inches tall.

Disclaimer. This is how formula is suppose to work. Actual size varies by manufacturers. For example Nitto drag radials will be a little narrower and shorter while there 555 street tire will almost be dead on with this formula. Also BFG KDW's seem to follow this as well. I don't know baout other tire brands.