Answer
We have two sources to calculate speed:
1. GPS, which uses the satellites to track longitude and latitude and then device distance by time. Our GPS is accurate within +- 3m (9ft). Curve-based logging is performed for both GPS position and GPS road speed to ensure the best fit on the map, as well as an accurate speed profile and idle time.
2. Engine Control Module (ECM), which may broadcast engine road speed or base speed on the rotation of the tire, so any changes to its diameter will affect the odometer accuracy.