TireFitCalc

Guide

How Speedometer Error Is Calculated

When you change tire size, your speedometer reading drifts. Here is exactly how the math works.

Key takeaways

  • Your speedometer doesn't measure ground speed directly — it measures how fast your wheels are turning, then converts that to MPH or KM/H using your factory tire's circumference.
  • When you fit larger tires, each rotation covers more ground, so the actual speed is higher than what's shown.
  • The error percentage equals (new diameter − original diameter) / original diameter × 100.

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Main explanation

Your speedometer doesn't measure ground speed directly — it measures how fast your wheels are turning, then converts that to MPH or KM/H using your factory tire's circumference.

When you fit larger tires, each rotation covers more ground, so the actual speed is higher than what's shown. Smaller tires do the opposite.

The error percentage equals (new diameter − original diameter) / original diameter × 100. Most countries allow up to ±3% before recalibration is recommended.

Frequently asked questions