GPS Speed Calibration Calculator

This calculator works by comparing the indicated speed with the speed a GPS device measures at the same moment. It returns the speedometer error and a correction factor.

1. Hold a steady speed on a flat, straight road, for example an indicated 60 mph.
2. Read the GPS speed, for example 58 mph.
3. Enter both values.
4. Read the error (+3.45%, reading high) and the factor (0.9667).

The formula is Error (%) = (Indicated − GPS) ÷ GPS × 100.

Worked example: indicated 70 mph with a GPS reading of 72.5 mph gives an error of −3.45% (reading low) and a factor of 1.0357.

GPS speed is most accurate at a steady speed with a clear sky view. Take 3 readings and average them.

The conversion table shows 10 sample readings with the error and correction factor each one produces. Six samples read high and 4 read low.

A factor below 1.0 means the speedometer reads high. A factor above 1.0 means it reads low, the usual result after fitting larger tires.

The GPS Speed Calibration Calculator is a tool that measures real speedometer error using a GPS reference instead of tire math. It captures every source of error: tire size, tread wear, pressure and the factory speedometer offset.

Tire math predicts error from the size change. GPS measures the total error directly, which makes it the best check after recalibration.

This calculator has 4 uses: post-recalibration checks, factory offset checks, worn-tire checks and data validation.

• Check recalibration: Confirm a tuner set the right value.
• Measure factory offset: See how far the stock speedometer reads high.
• Check worn tires: Measure the effect of tread loss.
• Validate data: Compare logged speed with GPS.