Standard telemetry sensors use your model's gear ratio and tire size to calculate its speed based on RPM. Using the Telemetry Expander and the GPS speed module, vehicle speed can be captured and displaced easily and more accurately. The Traxxas GPS speed module uses global positioning satellites to deliver precise, real-time speed information via the Traxxas Link App on your TQi Radio System with Docking Base.

http://traxxas.com/products/parts/te.../6551gpsmodule