[Al Skierkiewicz]

Guys,
Read some of the posts above before making guesses. The Hall effect sensors are easy tachometers that can tell you how fast a gear is rotating and how many gear teeth have passed by the front of it. Using two on the same gear, about 90% apart, can also give you direction information with some simple electronics or software interpretation. The accelerometer can be used only to give an indication of turning. It will give a large output for a bump or fast turn and a small output for a slow turn. Combine the data from all three and you will know that you have traveled "X" inches in some direction relative to your starting direction.
As a caveat, if your wheels are not firmly on the ground, counting gear teeth tell you nothing.