Car tires are measured through a standardized alphanumeric code molded into the sidewall, which indicates the tire's width, aspect ratio, construction, rim diameter, load index, and speed rating. For example, a common tire size like P225/65R17 101T breaks down as follows: The 'P' stands for P-metric (for passenger vehicles), '225' is the section width in millimeters, '65' is the aspect ratio (sidewall height as a percentage of width), 'R' signifies radial construction, and '17' is the wheel diameter in inches. The '101' is the load index (the maximum weight it can carry), and 'T' is the speed rating (the maximum speed it can sustain).
Beyond this primary size, other critical measurements include tread depth, measured in 32nds of an inch, and the UTQG (Uniform Tire Quality Grading) ratings for treadwear, traction, and temperature. Tread depth is vital for safety; new tires typically have 10/32" or 11/32" of tread, and most states legally require replacement at 2/32".
| Measurement Parameter | Example Value | Meaning & Standard |
|---|
| Section Width | 225 mm | The width of the tire from sidewall to sidewall. |
| Aspect Ratio | 65 | Sidewall height is 65% of the section width. |
| Rim Diameter | 17 inches | The size of the wheel the tire is designed to fit. |
| Load Index | 101 | Corresponds to a maximum load capacity of 1,819 pounds. |
| Speed Rating | T | Rated for a maximum speed of 118 mph (190 km/h). |
| Tread Depth (New) | 10/32" | The depth of the tread grooves on a new tire. |
| UTQG Treadwear | 400 | A comparative rating; a grade of 200 wears twice as fast as 400. |
| UTQG Traction | AA | The highest grade for a tire's ability to stop on wet pavement. |
| Overall Diameter | ~29.5 inches | The total height of the inflated tire without a load. |
Understanding these measurements is essential for selecting the correct replacement tires, ensuring vehicle safety, maintaining accurate speedometer readings, and preserving your car's handling characteristics. Always consult your vehicle's owner's manual or the placard on the driver's side door jamb for the manufacturer's recommended tire size.
