What do the numbers mean for an ATV tire size?

Inch size is the most widely used measurement in ATV tires, lawn tires, some older motorcycle tires and automotive tires. The example below is an ATV tire size 25x8-12 or 25x8.00-12.

25 = The overall height of the tire when mounted and inflated to recommended air pressure. In this example, the tire is 25 inches tall.

8 = The overall width of the tire when mounted and inflated to recommended air pressure. In this example, the tire is 8 inches wide.

12 = The diameter of the wheel that this tire will mount to. In this case it will be a 12 inch diameter wheel.

Also, if this size said 25x8R12 the "R" would indicate that the tire is of radial construction. Converted to metric scale, 25x8R12 looks like this: 205/80R12

205 = The width of the tire in millimeters when mounted and inflated to recommended air pressure. In this case it's 205mm wide. Divide this by 25.4 to convert to inches. The result is 8.07. This rounds off to 8 inches.

80 = The aspect ratio to the width of the tire when mounted and inflated to recommended air pressure. This specifies the sidewall height. In this case it's 80% of 205mm, which is 164. To convert the sidewall height to an overall diameter you must multiply that by 2 (which results in 328mm), then divide by 25.4 to convert to inches (which equals 12.913) and is then added to the wheel diameter (in this case, 12 inches). The result is 24.913 inches, which rounds off to 25 inches.

R = Radial construction.

12 = The diameter of the wheel that this tire will mount to. In this case it will be a 12 inch diameter wheel.

Load index and pressure codes

The number in the service description is the load index. This is the weight the tire is capable of handling when properly inflated. Most manufacturers will also spell out on the sidewall what that maximum load is so there is no guessing-you'll usually find it listed with the tire's maximum air pressure.

Besides under inflation, one of the biggest mistakes people make with their tires is to overfill them past the maximum level indicated on the sidewall. This leads to poor handling and premature wear. And be sure to measure pressure when the tire is cold. Measuring hot will skew the numbers.

Born on date and tire wear indicators

Another useful piece information on a tire's sidewall is its manufacturing date. Look on the side for a raised block with four digits; it's usually next to the U.S. DOT tire identification number. The first two indicate the week of its manufacture, and the last two are for the year. For example, 1702 would indicate the tire was manufactured in April, 2002 or the 17th week of 2002. Prior to 2000, there were only three digits, with the last one indicating the year.

Some tires may have raised triangles, or the letters TWI, to show where the tire wear indicators are in the tread. When these marks are equal to the tread, it's time for new tires.