Glossary
Plain-language definitions of the terms used across Sidewall Math.
- Aspect ratio
- Sidewall height as a percentage of section width (the 65 in 225/65R17). Lower numbers mean shorter sidewalls. Read more
- Extra load (XL)
- A passenger tire built to carry more load than a standard load tire of the same size, usually at a higher pressure.
- Flotation size
- A size such as 33x12.50R15 that states overall diameter and width in inches, followed by the wheel size.
- Load index
- A number after the tire size that stands for the maximum load per tire at its rated pressure. 91 means 615 kg (1,356 lb). Read more
- LT (light truck)
- Tires for light trucks, marked LT before the size, with their own load and inflation tables and a load range letter.
- Overall diameter
- The height of the tire from ground to top when not under load. It sets speedometer accuracy and axle height. Read more
- P-metric
- Tire sizes beginning with P, for passenger vehicles under U.S. standards.
- Placard
- The Tire and Loading Information label, usually on the driver's door post, listing original tire sizes and cold inflation pressures.
- Plus sizing
- Fitting a larger wheel with a lower-profile tire, aiming to keep the overall diameter close to the original. Read more
- Revolutions per mile
- How many times a tire turns in one mile. Calculated as 63,360 inches divided by the circumference; tire makers publish measured figures.
- Rim diameter
- The wheel diameter the tire fits, in inches (the 17 in 225/65R17).
- Rolling circumference
- The distance a tire travels in one revolution. Tire makers advise keeping it close to the original when changing sizes. Read more
- Section width
- The width of the tire from sidewall to sidewall, in millimeters in a metric size (the 225 in 225/65R17), measured on a specified rim width. Read more
- Service description
- The load index and speed symbol together, such as 91V, printed after the size.
- Speed rating
- A letter after the load index giving the maximum speed under test conditions. H is 210 km/h (130 mph); V is 240 km/h (149 mph). Read more
- Tire Identification Number (TIN)
- The code after DOT on the sidewall. Its last four digits give the week and year of manufacture. Read more
- TPMS
- Tire pressure monitoring system, required on U.S. cars, light trucks and vans from model year 2008. It warns when a tire is significantly underinflated. Read more
- UTQG
- Uniform Tire Quality Grading, the U.S. government system that grades treadwear, traction and temperature resistance. Read more
- Wear bars
- Raised bars in the bottom of the tread grooves. When the tread is level with them it is worn to 2/32 inch and should be replaced.