Tire Size Calculator
Calculate tire dimensions from tire size code. Compare two sizes for speedometer difference.
By Konstantin Iakovlev · Updated September 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. Results are estimates based on the information you provide and the assumptions described on this page.
How It Works
A tire's sidewall code packs its full geometry into a few characters, and reading that code correctly 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 205/55R16, 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: Fitting Larger Tires on a Pickup
- 1 Your pickup runs 275/60R20 tires and you're considering 285/65R20 for a more aggressive look and a little more ground clearance.
- 2 Enter 275/60R20 as the Tire Size and 285/65R20 under Compare With, without a P or LT prefix. The calculator converts each sidewall to inches and adds two sidewalls to the rim diameter.
- 3 275/60R20: sidewall 275 × 0.60 = 165 mm (6.50 in), overall diameter 20 + 2 × 6.50 = 32.99 in, circumference 103.65 in, 611 revolutions per mile. 285/65R20: sidewall 285 × 0.65 = 185.25 mm (7.29 in), overall diameter 34.59 in, 583 revolutions per mile. The diameter difference is +1.59 in.
- 4 Speedometer error: 32.99 ÷ 34.59 − 1 = −4.6%, so at a true 60 mph the speedometer reads about 57.2 mph; put the other way, when it shows 60 you are doing about 62.9 mph. Ride height rises by half the diameter difference, about 0.80 in. A change this large calls for recalibrating the speedometer.
Source: NHTSA — U.S. Dept. of Transportation · Last updated: September 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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