Your wife's GPS in her car does cheat. When you are underground you
lose all satellite reception and it then assumes that you are
continuing the last course. They call it dead reckoning. Automotive
GPS systems will typically watch your speed on the speedometer and also
your course vectors based upon any turns you have made, and tries to
keep you on course based upon that information. This is an important
feature in cars as many times in the middle of big cities satellite
reception can be very poor or non-existent if you are surrounded by
sky-scrapers. Then every time the GPS gains satellite reception back
it will correct itself again, the snapping back you see.

Slacker