Tire Size Codes
Authority: Tire and Rim Association (TRA) / ISO 4000
A tire code like P215/65R15 95H decodes as: P for passenger vehicle, 215 for section width in millimeters, 65 for aspect ratio as a percentage of width, R for radial construction, 15 for wheel diameter in inches, 95 for load index, and H for speed rating.
| Parameter / Specification | Standard Value | Unit |
|---|---|---|
| Type Prefix (P) | Passenger vehicle tire | - |
| Section Width (215) | 215 | mm |
| Aspect Ratio (65) | 65 | % of width |
| Construction Code (R) | Radial | - |
| Wheel Diameter (15) | 15 | in |
| Load Index (example: 95) | 1,521 lb / 690 kg max load | - |
| Speed Rating (example: H) | 130 mph / 210 km/h max | - |
History & Origin
Tire sizing wasn't always this precise. Through the mid-20th century tires were sized with simple numeric codes describing little more than overall width and wheel diameter, with an assumed aspect ratio close to 90-98% -- meaning the sidewall was nearly as tall as the tread was wide. As cars became lower and more powerful, tire makers began offering shorter, wider sidewalls for better handling, and Pirelli's introduction of a 70-series low-profile tire in the late 1960s helped popularize the 'aspect ratio' concept: a second number describing sidewall height as a percentage of tread width.
The modern alphanumeric code, standardized by the Tire and Rim Association in the US alongside equivalent ISO and ETRTO standards internationally, emerged in the 1970s as the P-metric system to accommodate the wider range of profiles and load capacities that smaller, front-wheel-drive cars demanded. It replaced the older numeric-only codes and remains the format used on tire sidewalls worldwide today.
Why the Aspect Ratio Matters
The aspect ratio number in a tire code isn't arbitrary -- it directly trades off ride comfort against handling precision. A higher aspect ratio (a taller sidewall relative to width) absorbs more road imperfection and gives a softer ride, which is why older sedans and comfort-oriented vehicles typically use tires in the 60-70 range. A lower aspect ratio (a shorter, stiffer sidewall) flexes less under cornering load, sharpening steering response, which is why performance cars and modern low-profile wheel setups favor ratios in the 30-45 range.
Because the aspect ratio is defined as a percentage of section width rather than a fixed dimension, the same aspect-ratio number produces a different actual sidewall height on a wide tire than on a narrow one -- one reason tire size substitutions need to be checked by overall diameter, not just by matching the aspect ratio number alone.