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.

Which tire codes can this calculator parse?

It accepts metric forms such as 225/45R17, P205/55R16, and LT275/65R18. It does not parse flotation sizes such as 33x12.50R15.

What does the 3% comparison threshold mean?

It is a screening threshold for highlighting a notable nominal-diameter change, not a universal manufacturer limit or a fitment guarantee. Use the size on the vehicle placard or another size recommended by the vehicle manufacturer.

Are these calculated dimensions exact?

No. They are nominal geometric estimates. Actual mounted dimensions vary with tire model, approved rim width, inflation, load, tread depth, and wear.

Tire Size Calculator

Enter a metric tire size like 225/45R17 to calculate nominal dimensions, or compare two sizes for ride-height and indicated-to-actual speed differences.

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.

Tire A Selected size

Difference (B vs A)

Speedometer estimate:

Diameter change: — beyond the ±3% comparison threshold; verify vehicle requirements

Indicated	Estimated actual	Difference

Cross-section Preview

Fitment note: These are nominal geometry estimates, not clearance or safety approval. NHTSA advises using the original tire size or another size recommended by the vehicle manufacturer. Check the driver-door Tire and Loading Information label or owner’s manual, and verify load index, speed rating, rim-width approval, inflation pressure, and clearance with a qualified tire professional.

Summary

Enter a metric tire size like 225/45R17 to calculate nominal dimensions, or compare two sizes for ride-height and indicated-to-actual speed differences.

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

Screen an alternate tire size before checking vehicle-specific clearance and rim requirements.
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.
Compare nominal codes while recognizing that measured dimensions vary by tire model and approved rim width.
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.

Frequently Asked Questions

Last updated: 2026-09-22 · Reviewed by Nham Vu