RollingSpecTire & Wheel Glossary
Plain-language definitions for the terms you'll run into on a tire sidewall or a wheel spec sheet. Each entry links back to the RollingSpec calculator that uses it, where applicable.
- Aspect Ratio #
- The tire's sidewall height expressed as a percentage of its section width. In a size like 225/65R17, the 65 means the sidewall is 65% of 225 mm (about 146 mm) tall. A lower aspect ratio (e.g. 40) means a shorter, stiffer sidewall; a higher one (e.g. 75) means a taller, softer sidewall.
- Backspacing #
- The distance from the wheel's mounting surface (hub face) to the back inner edge of the wheel rim. Combined with offset and width, it determines how much room the wheel leaves for suspension and brake components on the inboard side. See the Wheel Offset Calculator for a width/offset-to-backspacing conversion.
- Bolt Pattern (PCD) #
- Short for pitch circle diameter — the number of lug bolts/studs and the diameter of the circle they sit on, written like 5x114.3 (5 lugs on a 114.3 mm circle). A wheel's bolt pattern must match the hub exactly; it is unrelated to offset or width.
- Load Index #
- A numeric code on the tire sidewall (e.g. the 91 in 225/65R17 91V) that maps to a maximum load-carrying capacity per tire at full inflation, per an industry reference chart. Replacement tires should generally match or exceed the vehicle manufacturer's specified load index.
- Offset (ET) #
- The distance in millimeters between the wheel's centerline and its mounting (hub) surface, sometimes labeled ET (from the German Einpresstiefe). Positive offset places the mounting surface toward the outside of the wheel, tucking it inward; negative offset pushes the wheel outward. See the Wheel Offset Calculator to compare two offsets.
- Plus Sizing #
- The practice of fitting a larger-diameter wheel with a lower-profile tire so the overall tire diameter stays close to the vehicle's original spec (e.g. going from a 16-inch wheel/65-series tire to a 17-inch wheel/55-series tire — 'Plus One'). The goal is to preserve speedometer accuracy and rolling geometry while changing wheel diameter.
- Rolling Circumference #
- The distance a tire covers in one full revolution, equal to π × overall diameter. It determines how many revolutions per mile a tire makes, which is what a speedometer and odometer are calibrated against. Changing tire diameter changes rolling circumference and throws off both readings proportionally.
- Section Width #
- The first number in a tire size (e.g. the 225 in 225/65R17) — the tire's widest point from outer sidewall to outer sidewall, measured in millimeters, when mounted on the wheel width the manufacturer used for measurement. Mounting a tire on a narrower or wider wheel than that reference changes its actual measured width slightly.
- Sidewall Height #
- The vertical distance from the wheel rim to the tread, calculated as section width × aspect ratio. It's added twice (top and bottom) to the wheel diameter to get a tire's overall diameter. Shorter sidewalls generally mean sharper steering response and less cushioning over bumps.
- Speed Rating #
- A letter code on the tire sidewall (e.g. the V in 225/65R17 91V) indicating the maximum speed the tire is rated to sustain under its load index, per an industry reference table — for example S = 112 mph, T = 118 mph, H = 130 mph, V = 149 mph, W = 168 mph. It is not a recommended cruising speed.
- Tire Size Code #
- The full alphanumeric string molded into a tire's sidewall, e.g. "P225/65R17 91V." Reading left to right: an optional letter for tire class (P = passenger, LT = light truck), section width in mm, aspect ratio as a percentage, construction type (R = radial), wheel diameter in inches, then load index and speed rating.
- Wheel Diameter #
- The diameter of the wheel measured at the bead seat, in inches — the third number in a tire size (the 17 in 225/65R17). A tire's stated wheel diameter must match the wheel it's mounted on; this is not adjustable the way width and offset are between similar wheels.
- Wheel Width #
- The distance between a wheel's bead seats, in inches, which constrains which tire widths can be safely mounted on it (each tire size has a manufacturer-specified recommended wheel width range). Wheel width, together with offset, is what the Wheel Offset Calculator uses to estimate backspacing.
Want to put these terms into practice? Try theTire Size Calculator, theTire Diameter Calculator, or theWheel Offset Calculator.