Tire Size Comparator — Diameter & Speedometer Error
Compare two tire sizes (e.g. 205/55R16 vs 215/50R17): sidewall, overall diameter, circumference, difference in mm and %, and speedometer error at 100 km/h — all in your browser.
🔒 Runs entirely in your browser. Your input and output are never sent to a server or stored — fully private.
Diameter = rim × 25.4 + 2 × sidewall, where sidewall = width × aspect ÷ 100. A larger new tire makes your speedometer read low (actual speed higher than indicated). Click any value to copy.
About this tool
Thinking about plus-sizing your wheels? Enter two tire sizes — say 205/55R16 and 215/50R17 — and this calculator instantly compares their overall diameter, sidewall height and circumference, shows the difference as a percentage, and tells you how far your speedometer would be off after the swap. Because diameter affects speedo accuracy, effective gearing and fender clearance, the tool makes deviations obvious at a glance before you spend money on wheels or rubber. Every figure is computed locally in your browser from the numbers you type — nothing is uploaded or stored anywhere.
Frequently asked questions
Why should the new tire's overall diameter stay within about 3% of the original?
The overall diameter feeds into almost everything: the speedometer and odometer are calibrated for it, ABS and stability control compare wheel speeds based on it, and your effective gearing changes with it. A bigger deviation also risks the tire rubbing the fender or suspension at full lock or under load. The 3% figure is a widely used industry guideline, but the legal limits differ by country — Japan's shaken (車検) inspection, German TÜV approval and various other national rules each draw the line differently, so always check your local regulations before fitting.
Why does a speedometer usually read a little high, and how does a tire swap change that?
Regulations in most markets require that a speedometer never shows less than the true speed, so manufacturers deliberately calibrate it to over-read by a few percent with the factory tires. Fitting tires with a smaller overall diameter makes the wheels spin faster for the same road speed, increasing that over-read; a larger diameter reduces it and can eventually make the speedo under-read, which fails inspection in many countries. The error percentage this tool reports shows exactly how the indicated speed shifts relative to your actual speed.
How do I read a size like 205/55R16, and is anything I enter uploaded?
The first number is the tread width in millimetres (205), the second is the aspect ratio — the sidewall height as a percentage of that width (55%) — and the last is the rim diameter in inches (16); the R stands for radial construction. From those three values the tool derives sidewall height, overall diameter and circumference. Everything runs client-side in your browser: the sizes you type are compared on your device and are never uploaded, logged or shared with any server.