About the Tire Size Calculator
A tyre size such as 225/45R17 packs three numbers into one code: the tread width in millimetres, the sidewall height as a percentage of that width, and the rim diameter in inches. This tire size calculator decodes the code into real measurements: sidewall height, overall diameter, circumference and revolutions per kilometre or mile. If you are thinking of changing tyres or wheels, enter the new size too and the tool shows how much the diameter changes, how that affects the speedometer reading, and how ground clearance and sidewall height move.
Updated October 2026
How to use the Tire Size Calculator
- 1Read the size from your tyre sidewall, for example 225/45R17.
- 2Type it into the first box, with or without spaces or letters such as P or ZR.
- 3To compare, type the new size in the second box.
- 4Optionally enter a speedometer reading, such as 100, to see the real speed.
- 5Read the diameter change and decide whether the new size is close enough.
What the numbers on the sidewall mean
- Width: millimetres across the tread, such as 225.
- Aspect ratio: sidewall height as a percentage of the width, such as 45.
- Construction: R for radial.
- Rim: wheel diameter in inches, such as 17.
- After the size you will see a load index and speed rating, such as 94W, which this tool does not change.
Plus-sizing wheels
When owners fit larger rims, they keep the overall diameter close to the original by lowering the aspect ratio. For example, going from 225/45R17 to 235/40R18 changes the diameter by under one percent. Enter both sizes to see the exact figure, along with the width change, which matters for fender clearance, and the sidewall change, which affects ride comfort.
Related checks
Use the Speed Calculator to turn speed and time into distance, the Unit Converter to move between millimetres and inches, and the Fuel Economy Calculator, since larger or heavier tyres can change fuel use. This tool is a guide only. A tyre fitter can confirm which sizes are approved for your vehicle.
Features
- Width, sidewall, rim and overall diameter
- Circumference and wheel revolutions per km and mile
- Side-by-side comparison with a second size
- Speedometer error for a speed you choose
- Ground clearance and sidewall change
- Accepts P, LT and ZR style prefixes
- Runs in your browser
Frequently asked questions
How do I read a tyre size like 225/45R17?
The first number, 225, is the width of the tread in millimetres. The second, 45, is the aspect ratio: the sidewall height is 45 percent of the width. R means radial construction, and 17 is the wheel rim diameter in inches. So this tyre has a 101 mm sidewall and an overall diameter of about 634 mm.
How do I calculate tyre diameter?
Multiply the width by the aspect ratio and divide by 100 to get the sidewall height in millimetres. Double it, add the rim diameter converted to millimetres (inches times 25.4) and you have the overall diameter. For 225/45R17 that is 2 x 101.25 + 431.8, or 634.3 mm.
How much can I change tyre diameter?
Most advice is to stay within about 3 percent of the original overall diameter. Bigger changes affect the speedometer and odometer, gearing, ABS and traction control calibration, and may rub on the bodywork. Always check your vehicle manual or a tyre specialist before changing sizes.
Will a bigger tyre change my speedometer?
Yes. A larger diameter covers more ground per turn, so the car is going faster than the speedometer shows. A tyre 2 percent larger means that a speedometer reading of 100 is actually about 102. A smaller diameter makes the speedometer read too high.
What does a lower aspect ratio do?
A lower aspect ratio means a shorter sidewall. This usually gives sharper steering response and a sportier look, but a harsher ride and more risk of rim damage on potholes. To keep the overall diameter the same when moving to a larger rim, you need a lower aspect ratio.
Does this tool check load or speed ratings?
No. It only handles size and geometry. The load index and speed rating printed after the size also need to meet or exceed your vehicle's requirements, so check them on the placard or in the manual.