Tire Sizing ExplainedTires used to be sized in inches and were pretty easy to figure out. Drag racing tires and most large truck tires are still referred to by height and width, like 28x10.5, or 35x12.5, etc. Most passenger car tires use the metric sizing system, which lists the overall tire width from sidewall to sidewall, aspect ratio, and wheel diameter, in that order. It may also have the letter "R" for "radial" or "ZR" for "high-speed radial" in there (see sidebar "Sidewall Definitions" for more info).
Comparing tire sizes today is difficult due mostly to the aspect ratio, which is simply the ratio of sidewall height to width. The aspect ratio is actually a percentage of the tire's overall width and determines the sidewall height. For example: a 225/60R16 tire has a 225mm width and its sidewall is 60 percent of that (225 x 0.6 = 135mm). To determine the tire's overall height there is a little more math involved. The folks at Falken Tire were kind enough to give us some formulas to help out. Remember: To find inches you must first convert from millimeters. The conversion factor is 25.4 mm per inch.
225/60R16 Tire Example1. Calculating Width Width / 25.4 = Actual width in inchesEx. 225 / 25.4 = 8.86 inches wide
2. Calculating Overall HeightCalculate sidewall height by multiplying the aspect ratio times the width.Ex. 0.6 x 8.86 = 5.32 inches
3. Calculate total sidewall height; there is sidewall both above and below the rim.Ex. 5.32 x 2 = 10.64 inches
4. Calculate total overall height by adding the rim diameter in inches to the total sidewall height. Ex. 10.64 + 16 = 26.64 inches