justyourcalculatorTire Size Calculator
Compare two tire sizes and see the speedometer difference.
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Tire Size Calculator
Compare two tire sizes and speedometer error
Diameter difference
+1.72%
Within the common 3% guideline
- Speedometer shows 60 mph, actual speed97 km/h shows, actual 98.2 km/h
- 61.03 mph
- 225/45R17 diameter
- 24.97 in
- 235/40R18 diameter
- 25.4 in
- Ride height change
- 0.21 in
Calculated from the nominal size. Actual dimensions vary slightly by brand, tread wear and inflation.
How it was calculated
- Sidewall = width × aspect = 225 × 45% = 101.3 mm
- Diameter = wheel × 25.4 + 2 × sidewall = 431.8 + 202.5 = 634.3 mm
- Revs per mile = 1,609,344 mm ÷ (π × 634.3 mm) = 807.6
| 225/45R17 | 235/40R18 | Difference | |
|---|---|---|---|
| Overall diameter | 24.97 in (634.3 mm) | 25.4 in (645.2 mm) | 0.43 in (1.72%) |
| Section width | 8.86 in (225 mm) | 9.25 in (235 mm) | 0.39 in (4.44%) |
| Sidewall height | 3.99 in (101.3 mm) | 3.7 in (94 mm) | -0.29 in (-7.16%) |
| Wheel diameter | 17 in (431.8 mm) | 18 in (457.2 mm) | 1 in (5.88%) |
| Circumference | 78.45 in (1,992.7 mm) | 79.8 in (2,027 mm) | 1.35 in (1.72%) |
| Revolutions per mile | 807.6 | 794 | -13.6 (-1.69%) |
| Revolutions per km | 501.8 | 493.4 | -8.5 (-1.69%) |
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How the tire size calculator works
A tire size like 225/45R17 means a 225 mm section width, a sidewall 45% as tall as the width, and a 17 inch wheel. Overall diameter = wheel diameter + 2 × sidewall height.
Your speedometer counts wheel revolutions, so a taller tire makes you go faster than it shows. Actual speed = indicated speed × new diameter ÷ old diameter.
Most fitment guides keep the new overall diameter within about 3% of the original to avoid speedometer, ABS and clearance problems.
Using and checking your result
Published by JustYourCalculator. Check the units and assumptions above, and compare a known example before relying on the output. Calculations use browser arithmetic and may round displayed values.
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