How to Measure Tire Size and Wheel Diameter

Realize that the wheel and the tire are two different things.

Replacing wheels and tires is easy if you are planning on putting stock rims and tires back onto the vehicle. If you are looking to go with a bigger setup, making accurate measurements are crucial to ensure the stance and look you were hoping for on your vehicle.

Things You'll Need

• Tape measure

Instructions

1. Remove the tire from the rim. Flatten tire by removing all the air using an air valve removal tool. Take a screw driver and work at the edge of the tire and rim until you can move the edge over the rim. Continue until all the sidewall is over the edge of the rim using a lubricant such as hard grease or WD-40. Flip tire over and repeat process separating the tire from the rim in the same direction as before until the tire comes off.

2. Measure the rim or wheel diameter from the inside bead groove to the opposite side groove. This is the wheel diameter.

3. Determine the right tire for the rim. Other measurements that should be included are bolt pattern, backspace and offset. Four- or five-bolt patterns are typical. On a five-bolt pattern, measure distance from one bolt to an offset bolt. In other words, chose any of the bolts, skip the adjacent bolt, and measure to the next bolt. Backspace is measured from the face of the wheel to the back side edge of the wheel. Take two straight edges and measure across the rim bead and down the wheel face. This determines how much room the wheel has in the wheel well. Offset is how much of the wheel is toward the outside or inside from the face of the wheel. Positive offset is toward the outside, negative is toward the inside, and zero is over the rim.

4. Read the wall of your selected tire. A tire may read as a 225/50R17, for example. The "225" is the width of the tire in millimeters. The "50" refers to the aspect ratio, the ratio of sidewall height to width, and it is expressed as a percentage (in this case, 50 percent). So for this tire the sidewall is half as high (112.5 mm, or about 4.4 inches) as it is wide. The "R" stands for radial, and the "17" is the wheel diameter in inches. The tire should match the wheel.

Tips & Warnings

• Getting measurements right the first time will save you money and time in ordering replacements if you don't have enough clearance.

• Do not measure the tire with a tape measure while the tire is still on the rim. It will measure almost an inch larger than what it is.

References

• Turner Tools: Wheel and Tire 101-- Measuring and Fitting Properly
• Auto Amenity