Thinking of plus-sizing or downsizing your wheels? Compare any two tyre sizes to see the change in overall diameter, rolling circumference, sidewall height and speedometer error — and whether the new size stays inside the safe window.
Tyre Size Comparison & Speedometer Error Calculator
Compare your stock size with a new (plus-size or downsize) fitment. Enter each as width / aspect / rim, e.g. 205 / 55 / 16.
| Stock | New | Diff |
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Keep overall diameter within about ±3% of stock to avoid speedometer error, ABS/ESP confusion and clearance problems. A larger overall diameter makes the speedometer read slower than your true speed.
Why the ±3% rule matters
Keep the new tyre’s overall diameter within about ±3% of the original. Beyond that, your speedometer and odometer read wrong, the ABS/ESP and (on many cars) the cruise and transmission logic get confused, and you risk the tyre fouling the arch or suspension on full lock or compression. A larger overall diameter makes the speedometer under-read — you’re actually going faster than it shows.
Reading a tyre size
A size like 205/55 R16 means 205 mm wide, sidewall height 55% of the width, on a 16-inch rim. To keep diameter similar when you go up a rim size, drop the aspect ratio (e.g. 205/55 R16 → 225/45 R17).
Frequently asked questions
How much bigger tyre can I fit?
As a rule of thumb, stay within ±3% overall diameter. The calculator shows the exact percentage for your two sizes.
Will bigger tyres change my speedometer?
Yes — a larger diameter makes the speedo read low (you’re going faster than indicated). The tool shows your true speed at an indicated 100 km/h and 70 mph.
Do I need to tell the car about a new tyre size?
Most cars don’t need coding for a size within ±3%, but you should reset the tyre-pressure monitor and set the correct pressures for the new size.
Next: set the right pressures with our non-stock tyre pressure calculator and convert units with the tyre pressure converter.