Tyre size calculator
Compare two tyre sizes and see exactly what changes: diameter, width, sidewall height, circumference, ground clearance and speedometer error. The benchmark is ±3 % in diameter — within that, the size is considered safe.
Registration number
We fetch your car's tyre size from the MAX1 vehicle records and put it straight into the calculator.
Compare
Current size
As read on the sidewall: 315/40 R21
New size
As read on the sidewall: 235/60 R18
Not recommendedThe diameter changes by −5.9 %, which is well outside tolerance. The speedometer will be wrong, tachograph and ABS values will be off and the tyre may rub. Choose another size.
Current — 315/40 R21 New — 235/60 R18
| Measurement | 315/40 R21 | 235/60 R18 | Difference | Difference % |
|---|---|---|---|---|
| Overall diameter | 785.4 mm | 739.2 mm | −46.2 mm | −5.9 % |
| Tread width | 315 mm | 235 mm | −80 mm | −25.4 % |
| Sidewall height | 126.0 mm | 141.0 mm | +15.0 mm | +11.9 % |
| Rim size | 533.4 mm | 457.2 mm | −76.2 mm | −14.3 % |
| Circumference | 2,467 mm | 2,322 mm | −145 mm | −5.9 % |
| Revolutions per km | 405.3 | 430.6 | +25.3 | +6.3 % |
| Ground clearance (axle centre) | 392.7 mm | 369.6 mm | −23.1 mm | −5.9 % |
Speedometer
The speedometer is calibrated for the current size. With the new size it shows the following at these actual speeds:
Actual speed30speedo shows 31.9 km/h+1.9 km/h
Actual speed50speedo shows 53.1 km/h+3.1 km/h
Actual speed70speedo shows 74.4 km/h+4.4 km/h
Actual speed90speedo shows 95.6 km/h+5.6 km/h
Actual speed110speedo shows 116.9 km/h+6.9 km/h
What the numbers mean
DiameterOverall diameter of the tyre on the rim. It affects ground clearance, gearing and the speedometer. Stay within ±3 %.
Sidewall heightHeight of the rubber from the rim edge to the tread. A taller sidewall means a softer ride and better rim protection.
WidthWidth of the tread.
Circumference and revolutionsThe circumference determines how many times the tyre turns per kilometre — and therefore what the odometer records.
Ground clearanceHalf the change in diameter. A larger tyre raises the car but may rub against the wheel arch.
The ±3 % ruleWithin 3 % diameter difference, the speedometer, ABS and traction control stay within tolerance. Over 5 % is not recommended.
Please noteEven if a tyre should fit according to the calculation, that is no guarantee. Check with the vehicle manufacturer whether it is acceptable to use a size other than the one specified for your car.