Tyre Size Calculator
Compare two tyre sizes — diameter, circumference, and the speedometer error a change introduces.
A marking like 225/45R17 gives three numbers: the tread is 225 mm wide, the sidewall is 45% of that width tall, and the rim is 17 inches.
How the tyre size calculator works
A marking like 225/45R17 gives three numbers: the tread is 225 mm wide, the sidewall is 45% of that width tall, and the rim is 17 inches. Overall diameter is the rim plus two sidewalls.
Changing size changes how far the car travels per wheel revolution, and the speedometer does not know. A tyre 2% larger under-reads your speed by 2% — at an indicated 100 km/h you are actually doing 102.
The usual guidance is to stay within 3% of the original diameter. Beyond that, speedometer and odometer error becomes significant, and on many cars ABS and stability systems are calibrated to the original rolling radius.
Formula: diameter = rim + 2 × (width × aspect / 100)
Worked examples
| Inputs | Diameter change | Note |
|---|---|---|
| 225/45R17 to 245/40R18 | 2.98% | about 0.9% larger |
| A plus-size that goes too far | 7.248% | over 3% |
| An exact match | 0% | no change |
FAQFrequently asked questions
What does 225/45R17 mean?
225 mm tread width, a sidewall 45% of that height, on a 17-inch rim. R means radial construction.
How does a tyre change affect my speedometer?
A larger diameter travels further per revolution, so the speedometer under-reads. A 2% larger tyre means you are doing 102 when it shows 100.
How much size change is acceptable?
Within 3% of the original diameter is the usual guidance. Beyond that, speedometer error becomes significant and ABS calibration can be affected.
What is plus-sizing?
A larger rim with a lower-profile tyre, keeping overall diameter roughly the same. It sharpens steering response and worsens ride comfort.
Does a wider tyre always grip better?
Not necessarily, and it costs fuel economy and wet-weather performance. The placard size is chosen for good reasons.
Where these figures come from
- ISO 4000 / ETRTO — tyre designation and dimensions — the width / aspect ratio / rim marking this page decodes
- UK Vehicle Certification Agency — the official UK fuel and emissions data
Last checked: September 2026. Tyre dimensions follow the ISO metric marking standard; engine formulas are the standard textbook forms.