More like wheel speed. Not ground speed due to things like drifting or dyno, not air speed due to wind, not car speed as you can lock the brakes up and it'll show zero while sliding into the sunset at speed. GPS speed would be closer to "car speed", but there are still issues with z-axis movement.

That and if you're into modding cars, mustangs are a great choice due to the aftermarket selection with quality options.