Tire Size Calculator

A tire size like 225/45R17 hides the numbers that actually matter: how tall the tire stands and how far it travels in one turn. Enter the section width, aspect ratio and rim diameter and you get the overall diameter, sidewall height, circumference and revolutions per mile. Enter a second size and it compares the two, showing the difference in inches and percent, the new revolutions per mile, and the true speed when the speedometer reads 60 mph. Fitters usually keep a replacement within three percent of the original diameter, which the comparison flags for you.

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The size you type never changes: 225/45R17 is millimetres of width, a percentage for the sidewall and inches of rim on every passenger tyre in the world, and that is what is moulded on yours. This only chooses the units of the answers.
The first number on the sidewall. Always millimetres, in every market.
The number after the R. Always inches, in every market, even on an otherwise metric size.
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Results In
The size you type never changes: 225/45R17 is millimetres of width, a percentage for the sidewall and inches of rim on every passenger tyre in the world, and that is what is moulded on yours. This only chooses the units of the answers.
The first number on the sidewall. Always millimetres, in every market.
The first number on the sidewall. Always millimetres, in every market.
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