Tire Size Calculator
Compare two tire sizes: diameter, circumference and speedo error.
Enter your values
ⓘ Live results
Results
- New overall diameter
- 28.57 in
- Original diameter
- 28.52 in
- Diameter change
- 0.06 in (0.21%)
- Circumference (new)
- 89.77 in
- Revolutions per mile
- 706
- Speedo at indicated 60 mph
- 60.1 mph actual
- Fitment
- Within the usual ±3% safe window
Quick answer
Overall tire diameter = rim diameter + 2 × (section width × aspect ratio ÷ 100), converted to inches. A larger diameter makes the speedometer read low.
Diameter = rim + 2 × (width × aspect ÷ 100) ÷ 25.4
How to use the Tire Size Calculator
- 1Read your current size off the sidewall, e.g. 225/65R17.
- 2Enter width, aspect ratio and rim for the original and the new tire.
- 3Check the diameter change stays inside ±3% to keep the speedometer and ABS accurate.
Worked example
Using the values the calculator loads with:
Inputs
- Original width225 mm
- Original aspect ratio65 %
- Original rim17 in
- New width245 mm
- New aspect ratio60 %
- New rim17 in
Results
- New overall diameter28.57 in
- Original diameter28.52 in
- Diameter change0.06 in (0.21%)
- Circumference (new)89.77 in
- Revolutions per mile706
- Speedo at indicated 60 mph60.1 mph actual
- FitmentWithin the usual ±3% safe window
Frequently asked questions
How much tire size change is safe?
Most manufacturers accept an overall diameter within ±3% of stock. Beyond that, the speedometer, odometer, ABS and traction control all read incorrectly.
Does a wider tire change the diameter?
Yes — the sidewall height is a percentage of the width, so a wider tire with the same aspect ratio is also taller.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.