Will it fit?
Every other tyre calculator gives you two diameters and a percentage. This one gives you an answer — judged against the ±3% band vehicle makers actually calibrate to, with the speedometer consequence spelled out under the rule that applies where you drive.
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·both tyres drawn to one scale from their size designations; the ±3% ring is the industry fitment convention, not a legal limit·REVIEWED ETRTO STANDARD
- Sizes computed
- 1,945
- Fitment reports
- 816
- Data vintage
- None· it is arithmetic
- Sponsored placements
- Zero
Start here
What makes this different from the other four
There are several tyre calculators. Here is the specific thing each of them leaves out.
- A verdict, not a percentage. The ±3% convention is enforced in code and printed as an answer — within tolerance, marginal, or outside it — with the band drawn around the original tyre so you can see it.
- The rule where you live. UN ECE R39 is asymmetric: a speedometer may never read low. That single fact decides which direction of tyre change is risky, and no competitor mentions it.
- Drawings, not graphics. Every plate is a dimensioned engineering elevation rendered by the server, correct to one scale across the whole site, complete with JavaScript off.
- Stated limits. Geometry owns rolling diameter and nothing else. Every fitment page lists what it cannot tell you instead of implying a size string is a guarantee.
- No catalogue, no vintage. The figures derive from the designation, so they cannot go stale and nothing here is licensed from anyone with an interest in what you buy.
The sizes people look up most
The questions people arrive with
How do I know if a different tyre size will fit my car?
Stay within ±3% of your original rolling diameter and the speedometer, ABS and stability control stay inside the band the car was calibrated against. That is the part geometry can answer, and this site answers it for any pair of sizes. Rim width, arch clearance and load rating are the other half, and they live on the placard in your driver's door aperture.
What do the numbers on a tyre mean?
225/45R17 is three facts: 225 mm section width, a sidewall 45% as tall as that width, radial construction on a 17-inch rim. The 91V that usually follows is the load index (615 kg per tyre) and the speed symbol (240 km/h).
Will bigger tyres make my speedometer wrong?
Yes, by exactly the percentage change in rolling diameter, and in the direction that matters: taller tyres make the needle read low. Under UN ECE R39 a speedometer may read high by up to 10% + 4 km/h but may never read low at all.
Where do the figures on this site come from?
They are computed from the size designation itself — there is no catalogue and no licence behind them, which is why they have no vintage and cannot go stale. The method page prints every formula.
Every figure on this site is computed from the tyre size designation itself — see the method. The geometry here is exact; whether a tyre fits YOUR car is not something geometry can answer. Arch clearance, suspension travel, offset, load rating and your vehicle's own placard all decide that, and none of them are in a size string. The ±3% rule quoted throughout is an industry convention, not a law. Confirm any change against the tyre placard on your door pillar and with a tyre fitter before you buy.