How to use the Tire Size
- Enter your original tyre size as width, aspect ratio and rim diameter.
- Enter the new size you are considering.
- Read overall diameter, circumference and revolutions per mile for both.
- Check the speedometer error and keep it within about 3%.
How the calculation works
Sidewall height = width × aspect ratio ÷ 100, in millimetres. Overall diameter = rim diameter in millimetres + twice the sidewall height. So a 225/45R17 has a 101.25 mm sidewall and a 634 mm overall diameter. Circumference is π × diameter, and revolutions per mile is 1,609,344 ÷ circumference in millimetres.
Speedometer error is the diameter difference as a percentage: a larger tyre means the car travels further per revolution, so the speedometer under-reads and you are actually going faster than indicated. Keeping the change within about 3% preserves speedometer, odometer and ABS calibration accuracy. Beyond that, correction requires reprogramming. Also check load index and speed rating meet or exceed the originals, verify width fits the rim's recommended range, and confirm clearance at full lock and full suspension compression — larger diameters most often rub at the strut or inner arch liner rather than at rest.
Diameter = (rim × 25.4) + 2 × (width × aspect / 100) ; error % = (new − old) / old × 100Source: ISO metric tyre sizing convention and standard rolling-diameter geometry.
Worked example
Replacing 225/45R17 with 245/40R18.
- Old sidewall = 225 × 0.45 = 101.3; diameter = 431.8 + 202.5 = 634.3 mm.
- New sidewall = 245 × 0.40 = 98.0; diameter = 457.2 + 196.0 = 653.2 mm.
- Difference = 18.9 mm, or +2.98%.
- At an indicated 60 mph, true speed is about 61.8 mph.
A 2.98% increase — just inside the acceptable 3% limit, with the speedometer under-reading by about 1.8 mph at 60.
Frequently asked questions
How much size change is acceptable?+
Keep overall diameter within about 3% of the original to preserve speedometer and ABS accuracy.
Do bigger tyres make the speedometer read high or low?+
Low. You travel further per revolution, so true speed exceeds the indicated figure.
What does the aspect ratio mean?+
Sidewall height as a percentage of section width — 45 means the sidewall is 45% of the width.
Must I match the load index?+
Yes, meet or exceed the original load index and speed rating.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.