Your sizes
Use the sidewall markings, or start from an assumed factory size.
1Start from your vehicle—or enter the size
Start from your vehicleLoads the factory tyre size where it is published
Still on factory tyres? Choose your vehicle and we’ll load a common stock size. Check the tyre placard because trim and wheel packages vary.
2Current tyre
Example: 265/65R17 means 265 mm wide, sidewall height 65% of width, on a 17-inch wheel.
3New tyre
Fitment still matters. Nominal size does not confirm clearance, wheel width/offset, load and speed rating, spare-wheel fitment, braking or legality. Check the tyre placard, owner’s manual and current state rules.
Formula for the nerds
sidewall = width × aspect ratio ÷ 100
diameter = rim × 25.4 + 2 × sidewall
circumference = π × diameter
diameter = rim × 25.4 + 2 × sidewall
circumference = π × diameter
Actual speed equals indicated speed multiplied by new diameter divided by original diameter. Clearance change is half the diameter change; revolutions per kilometre are 1,000,000 divided by circumference in millimetres.