Tire Size Comparison Calculator
Free toolEnter the original and new tire size to see the diameter difference and whether it affects speedometer accuracy.
Difference between sizes
The difference is over 3%: this can affect speedometer and ABS accuracy. Check with a professional before fitting these tires.
How to use it
- Enter the current size in the format 205/55R16.
- Enter the new size you want to compare.
- Read the difference in diameter and the effect on the speedometer.
What it works out and how
Overall diameter is the rim diameter in inches multiplied by 25.4, plus twice the sidewall height, which is the width times the aspect ratio divided by 100. A 205/55R16 measures about 632 mm. The tool compares both diameters as a percentage and warns you if the difference exceeds 3%, the point beyond which the speedometer and driver aids are affected.
Frequently asked questions
What do the numbers mean?
In 205/55R16, 205 is the width in millimetres, 55 is the sidewall height as a percentage of the width and 16 is the rim diameter in inches.
How does it affect the speedometer?
With a larger wheel, the car travels faster than indicated. A 3% difference is about 3 km/h at 100 km/h.
Can I fit any size that physically fits?
No. Many countries only allow the sizes approved for the vehicle or authorised equivalents, and your insurer may require them too. Check before you buy.