[DaiPlusPlus]

> if the car's local sensors deviate from the gps data significantly, that should throw up a red flag to the car and driver.

This happens too often for benign reasons (and GPS hacking too rare) for it to be useful. The GPS system in my Ford Focus jumps around wildly if I start the car in an enclosed space before it gets a good lock, and it can be thrown-off under bridges and around tall buildings as it doesn't have A-GPS (does any car have A-GPS?). The worst is when a poor signal is combined with road-snapping - I've had a lot of trouble with off-ramps on partially submerged urban highways, the I-5 in downtown Seattle, for example.