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Speedometer Correction Calculator

Calculate actual speed after changing tire sizes.

Last updated: June 13, 2026

Calculator

mph
inches
inches

Results

Actual Speed64.6 mph

When speedometer shows 60 mph

Speedometer Error+7.7%
Speed Difference4.6 mph
Odometer Error+7.7%

Actual miles are more than shown

How to Use This Calculator

  1. 1Enter the speed shown on your speedometer.
  2. 2Enter your original tire diameter in inches.
  3. 3Enter your new tire diameter in inches.
  4. 4View your actual speed and error percentage.

Formula Explanation

When you change tire sizes, your speedometer becomes inaccurate because it calibrated for the original tire circumference.

Actual Speed = Speedometer Reading × (New Diameter ÷ Old Diameter)

Variables:

Speedometer ReadingSpeed displayed on dashboard
Old DiameterOriginal tire overall diameter
New DiameterNew tire overall diameter

Worked Examples

60 mph with 28" tires (was 26")

  1. 1. Actual = 60 × (28 ÷ 26) = 64.6 mph
Result: Actual speed: 64.6 mph (7.7% error)

Why Use This Calculator

  • Know your true speed for safety and avoiding tickets.
  • Calculate odometer error for accurate mileage tracking.
  • Essential after any tire size change.
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Tips & Best Practices

  • Keep error within ±3% for safe, legal driving.
  • Consider a speedometer recalibration for larger changes.
  • Odometer error mirrors speedometer error percentage.

Common Mistakes to Avoid

❌ Assuming the speedometer is still accurate after tire change.

✅ Always check — even a 1" diameter difference creates noticeable error.

❌ Not accounting for odometer error on resale.

✅ Odometer reads fewer miles with larger tires, which can affect vehicle value and maintenance schedules.

Frequently Asked Questions

Is it legal to have speedometer error?
Most jurisdictions require your speedometer to be accurate within a reasonable margin. Large tire changes (5%+) should be corrected for both safety and legal reasons.
How do I fix speedometer error?
Options include a speedometer recalibration device, a dealer reflash of the ECU, or a GPS speedometer app as a reference.
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Disclaimer: This calculator is provided for informational and educational purposes only. Results are estimates and should not be used as a substitute for professional advice. RediCalc.com makes no guarantees regarding the accuracy, completeness, or suitability of any calculations. Always consult a qualified professional before making financial, health, construction, or other important decisions.

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