Tire Size Calculator

Compare tire sizes, calculate speedometer difference, and find equivalent tire sizes.

Result

Fill in the fields above, then press Calculate.

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.

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.

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.
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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.