Tire Size Calculator
Diameter, sidewall, width and revolutions of a tire (tyre) size like 225/45R17 or 31x10.5R15, compared with a second size, and the speedometer error a change causes.
Result…
How it's calculated
Metric W/AR R D: sidewall = W × AR ÷ 100 (mm); diameter = D × 25.4 + 2 × sidewall Flotation D×W R rim: diameter = D in; sidewall = (D − rim) ÷ 2 Circumference = π × diameter Revolutions per km = 1,000,000 ÷ circumference (mm) Actual speed = speedometer × new diameter ÷ current diameter
Reads a tire size code from the sidewall, metric (225/45R17: 225 mm wide, sidewall 45% of the width, for a 17 in rim, with or without a P, LT, ZR or the load and speed rating) or flotation (31x10.5R15: 31 in tall, 10.5 in wide, 15 in rim), and gives its overall diameter, sidewall height, width, circumference and revolutions per km or mile. With a second size it shows the difference, and, because the speedometer counts wheel turns, the real speed when the speedometer shows a speed you choose. These are nominal figures from the size code; actual tires vary a little by maker and wear.
Your numbers, step by step
About the Tire Size calculator
This tire size calculator reads the size code on a tire's sidewall and turns it into real dimensions. A metric code such as 225/45R17 means a tire 225 mm wide whose sidewall is 45% of that width, on a 17 in rim; a flotation code such as 31x10.5R15 gives the overall diameter and width in inches directly. From either, it works out the overall diameter, sidewall height, width, circumference and revolutions per km or per mile. Enter a second size to compare them: because a speedometer counts wheel turns, a taller tire makes it read low and a shorter one makes it read high, and the calculator shows your actual speed for a reading you choose. Results are in millimetres or inches, with speeds in km/h or mph. To see what a speed change means over a trip, try the speed calculator.
How to use it
- Choose metric (mm, km/h) or imperial (inches, mph).
- Type the “Current tire size” exactly as it appears on the sidewall, for example 225/45R17 or 31x10.5R15.
- Type the “New tire size (optional)” to compare, or leave it empty to see one size.
- Enter a “Speedometer reading” and read the actual speed, the diameter difference and each tire's dimensions.
Worked example
A 225/45R17 tire has a sidewall of 225 × 45% = 101.25 mm and a diameter of 17 × 25.4 + 2 × 101.25 = 634.3 mm. A 235/40R18 has a 94 mm sidewall and a 645.2 mm diameter, 1.72% taller. With the larger tires, when the speedometer shows 100 km/h the car is doing about 101.7 km/h, and each tire turns about 493.4 times per km instead of 501.8.
Updated
Questions people ask
How do I calculate tire diameter from the size?
Multiply the width by the aspect ratio to get the sidewall height, double it and add the rim diameter in millimetres (inches × 25.4). For 225/45R17 that is 2 × 101.25 + 431.8 = 634.3 mm.
What does 225/45R17 mean on a tire?
225 is the section width in millimetres, 45 is the sidewall height as a percentage of the width, R means radial construction and 17 is the rim diameter in inches. Numbers and letters after it, like 91W, are the load index and speed rating.
Do bigger tires make the speedometer read slower?
Yes. A taller tire travels further each turn, so you go faster than the speedometer shows. The actual speed is the reading × new diameter ÷ current diameter.
How much tire size difference is acceptable?
Keep the overall diameter as close to the original as you can; the sizes approved for your vehicle are what count. Check the placard or manual, the load and speed ratings, and the clearance in the wheel well.
Why might the real tire measure differently?
The size code gives nominal dimensions. Actual diameter varies a little between brands and models, and with tread wear, pressure and load, so a measured tire can differ by a few millimetres.