Understanding Interchangeable Tire Sizes

by Jacob Burney

Tire aspect ratio and load capacity are determining factors in understanding interchangeable tire sizes. The old tire sizing system was based on aspect ratio, while current systems predominantly use load capacity. Tire conversion charts can help you determine which newer tires are compatible with your older car model.


The old numeric tire sizing system measured the relation of a tire's height to its width and incorporated an aspect ratio of 80 or 90. As of 2011, tire manufacturers use the alphanumeric and P-metric systems, characterized by the letters A to N. The letters measure load capacity, while additional numbers measure aspect ratio and the tire's overall size in inches.


Because your older tires use the numeric sizing system, it is crucial to convert that measurement to the alphanumeric or P-metric system. Current tires are measured by series in respect to their aspect ratio. Older cars had narrower tires and a smaller series tire, such as 70 or 80 series, should fit on your car. Your local auto shop or dealership should have tire conversion charts that can help you determine which new size tires you need for your car.


The letters and numbers on the side of your tires all measure different aspects of your wheels and car model. It is important to understand each of these markings to properly determine interchangeable tire sizes. If you have trouble deciphering all of the measurements, ask your local mechanic to help you select interchangeable tires for your car.

About the Author

Based in Washington, D.C., Jacob Burney has been writing professionally since 2005. He has written articles for "Broncos GameDay" magazine and the 2007 "South Pacific Games." He has also written several approved grant proposals. He is a former Peace Corps volunteer and holds a Bachelor of Science in business administration from Bucknell University with minors in philosophy and religion.

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