Tire Size Calculator

The default 265/70R17 is a common pickup and SUV size. Its sidewall is 265 × 0.70 = 185.5 mm, or 7.30 inches, so the whole tire is 17 + 2 × 7.30 = 31.6 inches tall and turns about 638 times per mile.

Tire code

Every modern passenger and light-truck tire carries a standardized size code on its sidewall, such as P225/55R17 97V. Reading it from left to right:

The optional first letter gives the service type. P means a P-metric passenger tire; LT means light truck; no letter means a European metric (ISO) tire, which has slightly different load tables; ST is a special trailer tire and T a temporary spare.

The first number is the section width, the widest point of the tire from sidewall to sidewall in millimeters when mounted on the specified rim width. The number after the slash is the aspect ratio, the sidewall height as a percentage of the width; a 55-series tire has a sidewall 55% as tall as it is wide. Lower numbers mean shorter, stiffer sidewalls with sharper handling and a firmer ride.

The letter R means radial construction, used by nearly all modern car tires; D or B denote older bias-ply designs. The next number is the diameter of the wheel, in inches, that the tire fits.

After the size come the load index and speed rating. The load index is a code for the maximum load each tire can carry at its maximum pressure; 97 means 730 kg (1,609 lb). The speed rating is a letter for the maximum sustained speed: S is 112 mph (180 km/h), T 118 mph (190 km/h), H 130 mph (210 km/h), V 149 mph (240 km/h), W 168 mph (270 km/h) and Y 186 mph (300 km/h). Replacement tires should match or exceed both figures on the vehicle placard.

Elsewhere on the sidewall you will find the DOT code, whose last four digits give the week and year of manufacture (2324 is the 23rd week of 2024), the maximum inflation pressure, which is not the recommended pressure, the UTQG ratings for treadwear, traction and temperature, and symbols such as M+S for mud and snow or the three-peak mountain snowflake for severe snow service.

How is tire size calculated?

The size code describes the tire's cross-section and the wheel, so its overall dimensions follow from simple arithmetic. Multiply the width by the aspect ratio to get the sidewall height, convert it to inches, add it twice, once above and once below the wheel, and add the wheel diameter. From the diameter come the circumference and the number of revolutions per mile, which is what the speedometer and odometer count. Real tires differ slightly from these nominal figures depending on the manufacturer, the rim width and how heavily the tire is loaded.

Diameter = wheel diameter + 2 × (width × aspect ratio ÷ 100 ÷ 25.4)

Width is in millimeters and the wheel diameter in inches; 25.4 converts millimeters to inches. Circumference = π × diameter.

What to consider when changing tire/wheel size?

Keep the overall diameter within about 3% of the original. The speedometer, odometer, anti-lock brakes, traction and stability control and the transmission's shift points are all calibrated to the original tire's revolutions per mile. A larger tire makes the speedometer read low: with a tire 3% taller, the car is doing about 61.8 mph when the speedometer shows 60, which can mean speeding tickets and a misleading odometer.

Plus-sizing, fitting a larger wheel with a lower-profile tire, keeps the diameter roughly constant while giving a sportier look and sharper steering, at the cost of a firmer ride, more road noise, higher tire prices and wheels that are easier to damage on potholes. Going the other way, smaller wheels with taller sidewalls, is popular for winter tires and off-road use because of the extra cushioning.

Check physical clearance. A wider or taller tire may rub on the suspension, fenders or inner liners at full steering lock or when the suspension compresses, and the wheel's offset and width must suit the tire. Make sure the new tires' load index and speed rating meet the vehicle's requirements, and keep the same size on each axle, or on all four wheels for all-wheel-drive vehicles, whose drivetrains can be damaged by mismatched diameters.

Finally, a larger or heavier tire and wheel increases rotating mass and rolling resistance, which slightly reduces acceleration, braking performance and fuel economy.

Frequently asked questions