Speedometer error calculator
How do different tires affect your speedometer?
Your speedometer is calibrated for the original tire diameter. A larger diameter means you are actually going faster than indicated; a smaller one, slower. The error equals the percentage change in diameter — computed instantly below.
Looks valid
Looks valid
Units
✓ Within toleranceDiameter +1.7% (+0.43 in) · US (~3%)
- Within tolerance · Diameter +1.7%.
- Speedo @ 60: 60 mph → 61.0 mph.
- Sidewall -0.29 in · Width +0.39 in.
Diameter+1.7%+0.43 in
Speedo @ 60+1.7%60 mph → 61.0 mph
Sidewall-0.29 inWidth +0.39 in
How this result is calculated
Computed from nominal tire-size geometry: diameter, circumference and speedometer error. It does not check wheel-arch, suspension or brake clearance.
Methodology- Diameter
- 24.97 in 25.40 in+0.43 in
- Sidewall
- 3.99 in 3.70 in-0.29 in
- Width
- 8.86 in 9.25 in+0.39 in
- Circumference
- 78.45 in 79.80 in+1.7%
- Revs / mile
- 808 794
- Speedo @ 60
- 60 mph 61.0 mph+1.7%
- Current —
225/45R17 - New —
235/40R18 - Grounded to the road: the axle rises +0.21 in.
Result report
225/45 R17 235/40 R18 · Within tolerance
Use this size when shopping: 235/40 R18
Geometric estimate. Always verify physical clearance (arches, suspension, brakes) before fitting.
Safe alternative sizes
| Size | Diameter | Speedo @ 100 | Use this |
|---|---|---|---|
−1″ 285/40R16 | +0.0% | +0.0% | |
OE 225/45R17 | +0.0% | +0.0% | |
+1″ 295/30R18 | -0.0% | -0.0% | |
+2″ 305/25R19 | +0.1% | +0.1% |
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What changed
- Reviewed deterministic geometry, load/speed references, sitemap inclusion and localized page shell.