I have yet to find a compass that actually works in a car without any problems.

I have tryed a few and soon as you swing then they turn out to be crap with huge amount of errors in them so as a rough guide they are okay but i have found them to be up to 50 degrees out at times. If you want a something of that sort buy a real cheapo GPS which can give you a better idea of direction you are heading in. But my favorite use of a small GPS is you can park you car in a huge car park set the GPS to where the car is and it will take you back to within a few feet, especially useful at the NEC in england.