Tyre size calculator: compare two sizes before you buy
This tyre size calculator (or tire size calculator, if you spell it the American way) compares the size on your car with the one you are thinking of fitting. Type both sizes as they appear on the sidewall and it shows the overall diameter, section width, sidewall height, circumference and revolutions per kilometre or mile of each, side by side and drawn to scale.
Changing tyre size changes how far the car travels for each turn of the wheel, so the page also works out the speedometer error at everyday speeds, checks the difference against the common ±3% guideline, compares load index and speed rating, and lists plus-size alternatives that keep the overall diameter close when you move to bigger or smaller wheels. It reads metric sizes (205/55 R16), P-metric and LT sizes, ZR sizes and inch or flotation sizes such as 31x10.50R15, in millimetres and km/h or inches and mph.
How to use the tyre size comparison
- Choose mm · km/h or inches · mph.
- Under Current tyre, type the size from your sidewall or door placard, or pick the width, profile and rim from the dropdowns.
- Enter the New tyre size the same way. Include the load index and speed symbol (for example 91V) to get the rating check.
- Read the diameter difference, the drawing, the size table and the speedometer table.
- Scroll to Plus-size alternatives, set the tolerance, and press Compare on any row to load it as the new tyre.
Tyre Size Calculator features
Every common size format
Metric, P-metric, LT, C (van), ZR, motorcycle and flotation inch sizes, with the load index and speed symbol decoded.
Speedometer and odometer error
True speed at six indicated speeds and the real distance behind every 10,000 on the odometer.
Scale drawing
Side and front views of both tyres on one ground line, with the current tyre’s outline over the new one.
Plus-size table
Standard sizes from one rim smaller to three larger within ±1, 2 or 3% of the current diameter.
Load and speed check
Flags a lower load index or speed rating, and the 1.10 load reduction when a passenger tyre replaces a light-truck tyre.
How tyre size is calculated
A metric size such as 205/55 R16 means a section width of 205 mm, a sidewall that is 55% of that width, radial construction (R) and a 16-inch rim. Following the ISO / ETRTO designation, the overall diameter is rim × 25.4 + 2 × width × aspect ÷ 100:
- Sidewall: 205 × 0.55 = 112.75 mm.
- Rim: 16 × 25.4 = 406.4 mm.
- Diameter: 406.4 + 2 × 112.75 = 631.9 mm (24.88 in). Circumference: π × 631.9 = 1,985 mm.
- Revolutions per km: 1,000,000 ÷ 1,985 = 503.7; per mile: 1,609,344 ÷ 1,985 = 811.
Flotation, P-metric, LT and ZR sizes
- 31x10.50R15 is an inch size: 31 in overall diameter, 10.5 in section width, 15 in rim, so each sidewall is (31 − 15) ÷ 2 = 8 in. That is about 787 × 267 mm, close to a 265/75R15.
- P marks a US P-metric passenger tyre and LT a light-truck tyre. The geometry is worked out the same way, but load ratings differ: US tyre makers (USTMA) say to divide a passenger tyre’s load by 1.10 when it replaces LT tyres.
- ZR marks a radial rated above 240 km/h; the speed symbol after the load index (W, Y) gives the actual limit, and a bracketed one such as (94Y) means above 300 km/h.
- C after the rim (195/70R15C) marks a commercial or van tyre; two load indexes such as 104/102 are for single and dual fitment.
- An old code with no aspect ratio, such as 185R14, is treated as 82%. Numeric sizes such as 7.50-16 are not supported.
The ±3% guideline and the speedometer
Tyre fitters commonly keep the new overall diameter within about 3% of the original. That figure is a guideline: the Tire Industry Association still publishes it (Modern Tire Dealer, September 2025), while US tyre makers point to the vehicle maker’s own tolerance. Bigger changes upset the speedometer, ABS and stability control calibration, gearing and clearance.
A speedometer counts wheel turns. With a taller tyre each turn covers more road, so it reads low. UN Regulation 39 (Rev.2, 2018), which car makers in the UK, EU and many other countries must meet, says a speedometer must never show less than the true speed, and at approval may read high by at most 10% + 4 km/h. Factory speedometers usually read a little high, which hides a small increase — but check against a GPS app after any change.
Tyre Size Calculator FAQ
How do I compare two tyre sizes?
Enter both sizes in the tyre size calculator above. It shows each tyre’s diameter, width, sidewall and revolutions, the difference in millimetres or inches and per cent, and the speedometer error.
What is the 3% rule for tyre size?
A common fitment guideline that the new tyre’s overall diameter should be within 3% of the original. It is not a law; your vehicle maker’s approved sizes take priority.
Will bigger tyres affect my speedometer?
Yes. A tyre 3% taller makes the speedometer read about 3% low: at an indicated 100 km/h you are doing about 103 km/h, and the odometer under-counts by the same share.
How do I convert 31x10.50R15 to metric?
Multiply by 25.4: 31 in diameter ≈ 787 mm and 10.5 in width ≈ 267 mm on a 15 in rim. The closest metric size is about 265/75R15; type it into the calculator to compare.
Can I fit a tyre with a lower load index?
Industry guidance says the replacement should not have a lower load index than the one on the vehicle placard. The calculator flags a lower index when both sizes include it.
How accurate is the tire size comparison chart?
It uses the nominal dimensions in the size code. Real tyres vary a few millimetres by make and tread wear, and the loaded rolling radius is smaller, so use the results for comparison and check the maker’s data for exact figures.