according to a professor of mine, a couple years ago someone built a senior project that used a 2-axis accelerometer and integrated to keep track of velocity and direction, and from that, build a map of where you went. he attached it to a bicycle, and left the engineering building and pedaled all over campus, and came back to where he started, and when he got back he plugged it into a computer and it showed his entire path, and shockingly enough it actually ended exactly where he had begun. I was pretty amazed at that, he must have used some pretty high-precision math.
if you used it in a car, it would be very interesting to see how close it could stay. like, drive for 10 minutes at various speeds and then come to a stop and see how close it reads to zero. perhaps with some sort of zeroing mechanism, so that, say, it zeros itself whenever your car is stopped, you could probably get decent accuracy.