Tire Size Calculator

How to use the tire size calculator

1. Type the Current tire size as printed on the sidewall, for example 225/45R17, P265/70R16 or LT245/75R16. Flotation sizes in inches such as 33x12.50R15 work too. The line under the field shows how the size was read.
2. To compare, type the New tire size (optional). Swap sizes (⇄) exchanges the two.
3. Read the Dimensions table: Section width, Sidewall height, Overall diameter and Circumference in millimetres and inches, the Revolutions per km and Revolutions per mile, and for two sizes the Difference and the Ground clearance change.
4. Choose the Speedometer units (km/h or mph) for the Speedometer error table, which shows the Actual speed when the speedometer reads 30 to 120 km/h or 20 to 70 mph.

A message tells you whether the overall diameters are within the usual ±3% guideline. Everything is calculated in your browser.

How to read a tire size

In 225/45R17 91V:

• 225 is the section width in millimetres.
• 45 is the aspect ratio: the sidewall height is 45% of the width.
• R means radial construction (ZR marks older high-speed ratings; D or “-” means bias ply).
• 17 is the rim diameter in inches.
• 91V is the load index and speed rating; they do not change the dimensions and are ignored.

A leading P (passenger) or LT (light truck) marks the US P-metric and LT-metric standards; the dimensions are computed the same way. Flotation sizes such as 33x12.50R15 give the overall diameter (33 in), the width (12.5 in) and the rim (15 in) directly.

The formulas

• Sidewall = width × aspect ratio ÷ 100
• Overall diameter = 2 × sidewall + rim × 25.4 mm
• Circumference = π × diameter
• Revolutions per km = 1,000,000 ÷ circumference in mm (per mile: 1,609,344 ÷ circumference)
• Actual speed = speedometer reading × new diameter ÷ current diameter

Example. 225/45R17: sidewall 225 × 0.45 = 101.25 mm, diameter 2 × 101.25 + 17 × 25.4 = 634.3 mm (24.97 in), circumference 1,992.7 mm, about 502 revolutions per km or 808 per mile. A 235/40R18 measures 645.2 mm, 1.72% taller: when the speedometer reads 100 km/h you are really doing 101.7 km/h, and at an indicated 60 mph about 61 mph.

Why the 3% guideline matters

The speedometer and odometer count wheel turns calibrated for the original size. A taller tire makes each turn cover more ground, so you drive faster than displayed and the odometer under-reads; a smaller one does the opposite. Beyond about 3%, the error becomes noticeable, and the change can affect ABS and stability control, gearing, and the clearance to fenders and suspension. A 245/45R18 in place of a 225/45R17, for example, is 6.84% taller and triggers a warning.

These are nominal dimensions. Real tires differ by a few millimetres between brands and shrink as the tread wears (new tread is about 8 mm deep).