Tire Size Calculator

Enter your current tire size and a size you are considering, exactly as printed on the sidewall, and the calculator compares their overall diameter, sidewall height, width and revolutions per mile. It also shows how fast you are really going when the speedometer shows a given speed after the change, since speedometers are calibrated to the original tire size. A to-scale drawing shows the two tires side by side. Use it when buying winter tires, changing wheel size or comparing options for a truck or SUV.

225/45R17: width 225 mm, sidewall height 45% of the width, radial construction, 17-inch wheel

The first number is the section width in millimeters. The second is the aspect ratio, the sidewall height as a percentage of the width. R means radial construction, and the last number is the wheel diameter in inches. A P or LT prefix marks passenger or light truck tires in the US. The load index and speed rating that follow, such as 94W, do not affect the size but must be suitable for your vehicle.

sidewall (mm) = width × aspect ratio ÷ 100

diameter (mm) = wheel diameter × 25.4 + 2 × sidewall

revolutions per mile = 1,609,344 mm ÷ (π × diameter)

For 225/45R17, the sidewall is 101.25 mm, the overall diameter 634.3 mm (24.97 in) and the tire turns about 808 times per mile. These are design dimensions; real tires vary slightly by brand, tread depth and pressure.

Going from 225/45R17 to 245/40R18, a common "plus one" upgrade to bigger wheels, the overall diameter grows by 2.98%. When the speedometer shows 60 mph you are actually doing about 61.8 mph, and the odometer under-records distance by the same proportion. The sidewall gets shorter, from 101 to 98 mm, which usually sharpens steering but gives a firmer ride.

• Many tire professionals advise keeping the overall diameter within about 3% of the original, to limit speedometer error and effects on anti-lock brakes and stability control.
• Check clearance to suspension parts and wheel arches at full steering lock and full suspension compression.
• Choose a load index and speed rating at least equal to those on the vehicle's tire placard, usually on the driver's door jamb.
• On all-wheel-drive vehicles, keep all four tires the same size, since mismatched diameters can strain the drivetrain.

Wider and heavier tires usually increase rolling resistance, which can lower fuel economy slightly, while larger diameters change the effective gear ratio and can make acceleration feel softer. A taller tire also means your trip computer under-reports distance, so calculated miles per gallon will read low unless the speedometer is corrected. For gear ratio effects, the gear ratio calculator can show how engine speed changes at a given road speed.

A speedometer counts wheel or transmission rotations and assumes a set distance per turn. Taller tires cover more ground per turn, so the speedometer reads low and you travel faster than shown; shorter tires make it read high. Worn tread also reduces the diameter slightly, which is one reason speedometers are often set to read a little high when new. Many vehicles can be recalibrated by a dealer or with a programmer after a size change.