first thing i would state as an assumption would be that the car is driving along the equator and that where the car is driving is a uniform feild, at whatever earths relative magnetic feild is at sea level, as you can loose a little of your potential at other points from the magnetic feild not being perfectly spherical, equally the earths magnetic feild is spotchy with some parts stronger and weaker across the surface,
driving west would mean it would be cutting feild lines head on, so the math is very easy from there, with velocity and length, still interesting,