Tire Size Calculator
Calculate tire dimensions from tire size code. Compare two sizes for speedometer difference.
By Konstantin Iakovlev · Updated April 2026 · Source: NHTSA — U.S. Dept. of Transportation
Overall Diameter
28.5"
Sidewall Height
5.8"
Revs per Mile
707
Tire: 225/65R17
| Section Width | 225 mm |
| Aspect Ratio | 65% |
| Rim Diameter | 17" |
| Sidewall Height | 146.3 mm (5.76") |
| Overall Diameter | 28.52" |
| Circumference | 89.58" |
| Revolutions per Mile | 707 |
Use the Tire Size Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.
Disclaimer: This calculator is for informational purposes only and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.
How It Works
A tire's sidewall code packs its full geometry into a few characters, and reading that code correctly in 2026 is what keeps a replacement set fitting properly, the speedometer honest, and handling predictable. Stray too far from the original specification and you invite uneven wear, degraded cornering, and in some cases a voided vehicle warranty.
From a code such as P205/55R16, the math derives overall diameter, sidewall height, and circumference using the section width, aspect ratio, and rim diameter. The speedometer comparison works by setting one tire's circumference against another's: the difference in rotations per mile maps directly onto how far off your indicated speed will read against your true speed.
Confirm your current size by reading the sidewall itself rather than trusting the owner's manual, since a prior owner may have fitted something different. A circumference gap that looks trivial as a percentage compounds into a meaningful speedometer error across many miles of driving. And never pair a radial with a bias-ply tire on the same axle, because the mismatch can wreck handling outright.
Example: Upgrading Tires on a 2026 Ford F-150 Lariat
- 1 Your 2026 Ford F-150 Lariat currently has P275/60R20 tires. You're considering upgrading to a larger P285/65R20 tire for a more aggressive look and slightly increased ground clearance.
- 2 Input P275/60R20 as 'Tire 1' and P285/65R20 as 'Tire 2' into the calculator. The calculator will determine the overall diameter, sidewall height, and circumference for each tire. It will then calculate the percentage difference between their circumferences.
- 3 The calculator reveals that the P275/60R20 tire has an overall diameter of 32.99 inches and a circumference of 103.65 inches. The P285/65R20 tire has an overall diameter of 34.61 inches and a circumference of 108.79 inches. This results in a 4.96% increase in circumference for the larger tire.
- 4 This 4.96% increase means that when your speedometer reads 60 MPH, your actual speed will be approximately 62.98 MPH. You should consider recalibrating your speedometer to ensure accurate readings, especially for legal compliance and safety. The larger tires will also raise your truck's ride height by approximately 0.81 inches.
Source: NHTSA — U.S. Dept. of Transportation · Last updated: April 2026
Frequently Asked Questions
How do I read a tire size like 225/65R17?
What happens if I use a different tire size?
Can I put wider tires on my car?
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