Tire Size Calculator
Compare tire sizes, calculate speedometer difference, and find equivalent tire sizes.
A tire size calculator decodes metric tire designations into physical dimensions such as diameter, circumference, and sidewall height, and compares two tire sizes for fitment and speedometer accuracy.
Examples
225/45R17
Comparing 225/45R17 to 235/40R18
Frequently Asked Questions
How much tire size difference is acceptable?
What does the aspect ratio mean?
Quick Tips
- •Stay within 3% of your original tire diameter to avoid significant speedometer error and drivetrain stress.
- •A lower aspect ratio improves handling response but reduces ride comfort and increases susceptibility to pothole damage.
- •When upsizing wheels, decrease the aspect ratio proportionally to keep overall diameter similar.
A tire size calculator decodes metric tire designations into physical dimensions such as diameter, circumference, and sidewall height, and compares two tire sizes for fitment and speedometer accuracy.
How to Use This Calculator
Enter a tire size in standard format (e.g. 225/45R17). Optionally enter a second tire size to compare dimensions and calculate speedometer error. The first number is tread width in mm, second is aspect ratio (sidewall height as % of width), and the last number is rim diameter in inches.
Understanding the Formula
Sidewall = Width x (Aspect/100). Overall Diameter = Rim + 2 x Sidewall (converted to inches). Circumference = pi x Diameter. Revs/Mile = 63,360 / Circumference.
Examples
225/45R17
Sidewall: 225 x 0.45 = 101.25 mm (3.99 in). Diameter: 17 + 2 x 3.99 = 24.97 in. Circumference: 78.44 in. Revs/mile: 808.
Comparing 225/45R17 to 235/40R18
Tire 1 diameter: 24.97 in. Tire 2 diameter: 25.40 in. Difference: +0.43 in (1.7%). At 60 mph indicated, actual speed would be ~61 mph.
Frequently Asked Questions
How much tire size difference is acceptable?
Most experts recommend staying within 3% of the original tire diameter. Larger changes can affect speedometer accuracy, ABS, traction control, and ride quality.
What does the aspect ratio mean?
The aspect ratio is the sidewall height as a percentage of the tread width. A 225/45 tire has a sidewall that is 45% of 225 mm = 101.25 mm. Lower aspect ratios mean shorter sidewalls and sportier handling.
Assumptions & Limitations
- Tire dimensions are calculated from nominal sizes; actual measurements vary slightly with inflation pressure and load.
- Only metric tire format (e.g. 225/45R17) is supported; flotation and LT-numeric formats are not parsed.
- Speedometer error is based purely on circumference difference and does not account for tire wear or deformation at speed.