There seems to be some confusion over what the numbers in a tire size signify. It's actually easy to break down, and with a little math you can translate the whole thing into numbers that mean something to us consumers!

As an example, let's work with 165/90/R15. The 'R15' means it's a Radial tire on a 15-inch rim. Pretty simple so far.

The '165' means the tire is 165 millimeters wide, or about 6-1/2 inches.

The '90' refers to the ratio of the height of the sidewall as compared to the width of the tire, or 90% of 165 mm. 165 ÷ 90% (or 165 X .9) = 148.5 mm, or about 5-15/16 inches high.

The lower the ratio number, the lower the sidewall: a 165/50R15 tire would have a sidewall height of only about 3-3/8 inches!

You can also calculate the circumference of the tire in order to figure out what size will keep you within reason on your odometer:

Figure the radius of the wheel and tire assembly by taking half the wheel size (15 ÷ 2 = 7.5 inches) and adding the tire height (6.5 inches in our example). You now know this tire will have a radius of 14 inches.