aspect ratio

A tire's aspect ratio is the sidewall height written as a percentage of the tire's width, so 65 in P215/65R15 means 65 per cent of 215 mm, not 65 mm.

An aspect ratio is the middle number in a tire size, and it is a percentage, not a measurement. In P215/65R15 the 215 is the section width in millimeters, the 65 is the aspect ratio, and the 15 is the wheel diameter in inches. The sidewall height is therefore 65 per cent of 215 mm, or about 140 mm — not 65 of anything. Read the two units together and the code stops being a code: width in millimeters, height as a share of that width, wheel in inches.

Because it is a ratio, the same number means a taller sidewall on a wider tire, which is why plus-sizing keeps the overall diameter roughly constant when the wheel grows and the aspect ratio drops.

The handling claim follows from the shape. A lower aspect ratio means a shorter, stiffer sidewall: less cushioning over bumps, less sidewall flex under cornering, and quicker steering response. A higher ratio means a taller, softer sidewall: a more forgiving ride and slower, vaguer response. Neither is better in the abstract; they are two trades of the same coin.

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