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How to Measure Car Tire Sizes

Updated March 23, 2017

When purchasing a set of tires, it is important to know the tire sizes that can fit your vehicle. Tires that are too big, small or not explicitly designed for your vehicle can be a potential safety hazard. In addition, the wrong tire size will have a negative impact in your car's overall performance. Several sections of a tire must be measured in order to get an overall size.

Locate the tire size markings on the sidewall of the tire. An example of a tire size marking is: P205/40R16 82H; the markings are usually printed in white and clearly displayed on the tire.

Decode the tire size markings on your current tires. Using the example above, "P" refers to tires made for passenger vehicles. The number 205 refers to the width of the sidewall, in millimetres. The number 40 is the ratio of the tread width to the sidewall height, and is also known as the "aspect ratio." The "R" stands for "radial," 16 refers to the size of the wheel/rim, 82 is the maximum load capacity and "H" represents the speed that the tire can handle.

Compare the current tire size markings to the new tire markings. Take a copy of your tire markings to the store. Only purchase new tires with markings that match your current tire size markings to ensure that they fit your vehicle.

Use the car jack to prop up the car and remove a wheel from the car. In order to take accurate measurements of the tire, there must not be any weight on the tire, as it can distort the results. Properly inflate the tire and place the wheel on a flat surface, facing up.

Measure the overall width and the section width of the tire, using a tape measure. Measure the distance from the outside of one sidewall to the outside of the sidewall on the opposite end. The difference between the section width and the overall width is the size of the section width, and it does not include the manufacturer's markings or any protective ribs in its measurement.

Place the end of the tape measure on the outer thread surface and extend it to the intersection where the tire meets the wheel. This measurement is the section height of the tire. Measure the distance between the lower and upper thread surfaces to get the diameter of the tire. To get the thread width, measure the distance from the outer edge of the thread to the inner edge. Record all of the measurements.

Place the tire back onto your vehicle after you have finished measuring the tire size.

Things You'll Need

  • Tape measure
  • Jack
  • Lug wrench
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About the Author

An avid technology enthusiast, Steve Gregory has been writing professionally since 2002. With more than 10 years of experience as a network administrator, Gregory holds an Information Management certificate from the University of Maryland and is pursuing MCSE certification. His work has appeared in numerous online publications, including Chron and GlobalPost.