What does a tire size like 225/45R17 mean?

225 is the section width in millimeters, 45 is the aspect ratio (sidewall height is 45% of 225 mm = 101.25 mm), R means radial construction, and 17 is the rim diameter in inches.

How is overall tire diameter calculated?

Overall diameter = (sidewall height × 2) + rim diameter converted to mm. Sidewall height = (section width × aspect ratio) / 100. The rim diameter in inches is multiplied by 25.4 to convert to mm.

What is revolutions per mile and why does it matter?

Revolutions per mile = 63,360 inches ÷ tire circumference in inches. A bigger tire has a larger circumference, so it turns fewer times per mile, making your speedometer read lower than your true speed.

Will changing tire size affect my speedometer?

Yes. A tire with a larger overall diameter covers more ground per revolution, so your speedometer (which counts wheel rotations) will under-read your actual speed. This calculator shows the circumference difference to help you quantify the effect.

Can I use this for metric, LT, or trailer tire sizes?

Yes, as long as you know the section width (mm), aspect ratio (%), and rim diameter (inches). Most passenger, light truck, and metric tire sizes follow the same formula. ST (special trailer) and flotation sizes use the same math.

What is a safe diameter difference between two tire sizes?

Most manufacturers recommend keeping the overall diameter within 3% of the stock size to avoid drivetrain stress, clearance issues, and significant speedometer error.

Tire Size Calculator

Name: Tire Size Calculator
Availability: InStock
Author: Nham Vu

Enter a P-metric tire size like 225/45R17 and instantly see the actual diameter, sidewall height, circumference, and width in both millimeters and inches.

Tire Size Input

Paste full size (e.g. 225/45R17)

Paste the tire code and we will fill the fields below automatically.

Width (mm)

Aspect %

Rim (in)

Enter a tire size and click Calculate to see dimensions.

Summary

Enter a P-metric tire size like 225/45R17 and instantly see the actual diameter, sidewall height, circumference, and width in both millimeters and inches.

How it works

Enter your tire's section width in millimeters (e.g. 225).
Enter the aspect ratio — the sidewall height as a percentage of the section width (e.g. 45).
Enter the rim diameter in inches (e.g. 17).
Click Calculate to see all dimensions: diameter, sidewall height, circumference, and revolutions per mile.
Switch between metric and imperial tabs to view results in your preferred units.
Use the compare mode to evaluate two tire sizes side by side.

Use cases

Check whether an aftermarket tire fits your wheel without rubbing the fender.
Compare OEM and upgraded tire sizes before buying.
Verify that a tire swap will not significantly alter your speedometer reading.
Calculate circumference to estimate revolutions per mile for fuel economy calculations.
Confirm that two different-brand tires with the same P-metric code are truly the same size.
Plan a lift or lowering kit by understanding how a larger or smaller tire changes ride height.
Evaluate the trade-off between a wider low-profile tire and a narrower high-sidewall tire.
Cross-reference metric tire sizes with older inch-based tire designations.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu