Tire Size Calculator
Decode a tire size like 225/45R17 into its real diameter, sidewall height, circumference, and revolutions per mile, and find the speedometer error when you fit a different size. Calculates in your browser.
Original tire diameter
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Show the math
A tire size like 225/45R17 encodes the diameter:
sidewall = width × aspect%
diameter = rim × 25.4 + 2 × sidewall
true speed = indicated × (new diameter ÷ old diameter)
Your speedometer is calibrated for the original tire. A taller replacement makes you actually travel faster than the dial shows; a shorter one, slower. All math runs locally in your browser.
What this does
A tire size calculator decodes a size like 225/45R17 into its real diameter, sidewall height, circumference, and revolutions per mile, and finds the speedometer error when you fit a different size.
How it works
In 225/45R17, 225 is the section width in mm, 45 is the aspect ratio (sidewall as a percent of width), and 17 is the wheel diameter in inches. These give the overall diameter and circumference.
How to use it
- Enter your original tire size.
- Enter a new tire size to compare.
- Read the diameter and circumference.
- See the speedometer error.
Understanding your result
A taller tire travels further per revolution, so you actually go faster than the dial reads; a shorter tire reads high. Revolutions per mile is 63,360 inches ÷ circumference.
Example
A 225/45R17 tire is about 24.97 inches in overall diameter.
Sources & methodology
Last updated .
Frequently asked questions
How do I read a tire size?
In 225/45R17, 225 is the section width in mm, 45 is the aspect ratio (sidewall height as a percent of width), and 17 is the wheel diameter in inches. The tool turns these into the overall diameter.
How does tire size affect my speedometer?
Your speedometer is calibrated for the original tire. A taller tire travels further per revolution, so you actually go faster than the dial reads; a shorter tire reads high. The tool shows the true speed.
What are revolutions per mile?
How many times the tire turns to cover a mile (63,360 inches ÷ circumference). It affects gearing, speedometer, and odometer readings.
Is my data saved?
No. All calculations happen locally in your browser.
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