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Auto & Vehicle planning

GPS Speedometer Calibration

Calibrate against observed GPS measurements rather than predicting error from a tire-size change.

Enter measured or assumed values

Method and units

Speed factor = GPS speed ÷ displayed speed. Corrected display estimate = displayed speed × factor. Distance factor = GPS-measured distance ÷ indicated odometer distance; corrected odometer = indication × distance factor. Speed and distance factors are measured separately.

Worked example

At 60 mph displayed and 58 mph GPS, factor is 0.9667, so the display is 3.33% high. If 100 odometer miles accompany 98 GPS miles, distance factor is 0.98 and 100 indicated miles correspond to 98 measured miles.

Supported inputs and interpretation

Speeds 0.1–200 mph; distances 0.1–100,000 mi. Results are rounded for display only; internal calculations retain full JavaScript precision. Entered measurements and assumptions determine the answer.

References and limitations

The equations here use the stated unit conversions and geometric or accounting relationships; no empirical vehicle limits or manufacturer presets are supplied.

Use stable GPS reception, matched time/distance windows and a safe controlled measurement process. GPS speed can be noisy and route distance uncertain. This is not a tire-size model, certification, recalibration procedure or permission to alter instruments.

Related calculators

This planner performs the stated arithmetic; it is not a vehicle inspection, engineering approval, manufacturer's recommendation or professional review.

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