The tire sizing used for DOT-legal competition and track day competition and track day tires share the same P-metric or Euro-metric tire sizing systems used for high-performance passenger car tires. However, tire size formats used for Non-DOT-legal circuit racing slicks and rally motorsports tires have not standardized by industry tire & wheel associations. This has resulted in race tire manufacturers adopting several different formats for communicating their tire sizes.
Most Non-DOT motorsports tires identify tire width (either tread width or section width as selected by the tire manufacturer), overall tire diameter, and wheel diameter. Non-DOT motorsports tires feature tire sizing formats more similar to the industry's Flotation Light Truck tire sizing standards, such as 35x12.50R17, than they do to metric passenger car tire sizes.
American racing tire manufacturers tend to present tire dimensions in inches, while European and Asian manufacturers provide their racing tire dimensions in millimeters or centimeters. Here are some examples currently used by several racing tire manufacturers:
Manufacturer Measuring Unit Format / Example
Note: Most racing tire manufacturers list their non-DOT motorsports tires' tread widths in their tire size formats, while Pirelli lists its tire's section widths. Since a tire's section width is always wider than its tread width, the manufacturer's choice of representing overall tire width based on tread width or section width can make a significant difference regarding suspension, fender clearances and calculating equivalent passenger car sizes.
Since multiple units of measurement are commonly used, it's important to confirm the conversion factors used to calculate equivalencies between the multiple unit types.
When comparing tires sizes to manufacturers' tire dimensions, the manufacturers' assigned sizes round off both imperial and metric dimensions.
While circuit racing slicks and rally motorsports tires have more squared-off shoulders than high-performance street tires, there is a significant difference between a slick tire's tread width and its section width when mounted on its recommended-width wheel. We'll use the 27/65-18 (based on tread width) size to calculate dimensions when mounted on its recommended 11-inch-wide wheel and properly inflated.
Size Described in Centimeters, Millimeters, Inches
When all of these three race tire size formats based on tread width are essentially equivalent in their physical dimensions.
To identify an equivalent passenger car metric tire size, requires using the provided values to calculate the closest high-performance P-metric or Euro-metric size. Begin by calculating the tire Section Height, then dividing it by the tire Section Width to identify the tire's effective Aspect Ratio. Section Height and Aspect Ratio can be calculated as follows:
(Inflated Diameter minus Wheel Diameter) ÷ 2 = Tire Section Height
(25.6" -18")÷ 2 = 3.8"
Section Height divided by Section Width = Aspect Ratio Series
3.8"÷11.6"=0.33
Therefore, a 27/65-18 racing tire size is the metric-size equivalent of a tire with a 295 Section Width and 35 Series Aspect Ratio on an 18-inch diameter wheel, or a 295/35R18 295/35R18 high-performance passenger car tire size.
Due to the multiple race tire size formats being used, initially selecting Non-DOT circuit slicks and rally motorsports tires often requires research, calculations, and seeking expert advice.
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