Tire sizes can be confusing to the consumer since the information molded into the side of the tire seems to be a cryptic string of numbers that only a tire technician can decipher. That isn't the case, of course. The letter at the beginning of the numbers tells what type of service the tire is intended for: P for passenger, LT for light truck, T for a temporary spare--the donut in the trunk. ST is for boat and utility trailers. The first three numbers are its "section width" and the second two are its "section height." The rest deals with the tire's construction, the rim size it fits on and its speed and load carrying capabilities.

**Step 1** Look at the sidewall of your tire and find a number that looks like this: P 225/40 R16 91S. The "section width" of the tire is the first three numbers, in millimeters. In this case, the section width is 225mm.**Step 2** Look at the last two numbers in the sequence "225/40." The 40 means that the height of the tire, measured from the tread of the tire to the rim of the wheel is 40 percent (0.40) of the section width (225mm). Use your calculator to work out the height of the tire from rim to tread: 0.40 x 225 = 90mm. This height is referred to as (a) the section height, (b) the series of the tire, (c) the tire's profile or (d) the tire's aspect ratio, depending on who's speaking. A larger number means a higher sidewall.**Step 3** Calculate the size in inches by dividing the section width by 25.4, the number of millimeters in an inch: 225 / 25.4 = 8.86 inches. The section height is then determined by multiplying 8.86 inches by 0.40 (forty percent): 8.86 x 0.40 = 3.54 inches.

You can determine the overall height of the tire, from the road surface to the tread on top of the tire, by multiplying the section height by two, and adding the rim size: 3.54 x 2 = 7.08 + 16 (remember that the two numbers following the letter R tell you the rim size. A tire marked "P 225/40 R16 91S" uses 16 inch rims) = 23.08 inches.**Tip**: The last two numbers in the example, "P 225/40 R16 91S," are 91S and describe the load index and speed rating of the tire. The first two numbers in 91S refer to the load index. The higher the number, the more weight the tire can carry. Most car- and light-truck tires are rated between 70 and 110. In this case, a tire rated 91 can carry about 1,356 pounds. The letter following the numbers is the speed rating. Modern tires are rated from 75 mph to 149 mph. An "S" rating is a mid-range tire capable of 112 mph, according to TireRack.com.**Items you will need**: Calculator