[Bongle]

You can't do much about the constantly-increasing distance. You see the same thing with the gyro: it has a little bit of error in it, so the robot constantly drifts a little bit. You can work on your integration routines to make them a bit more accurate, but you'll always have that little error.

To fix it:
-If you ever have a point where you KNOW the robot isn't moving (encoder speed on a freely turning wheel = 0), then you can zero out the velocity and start integrating from there again
-Use encoders on a free wheel to track distance, rather than accelerometers. Encoders do not require any integration, and so have much less error. According to QA responses, you are allowed having an unpowered wheel touch the ground that isn't a rover wheel, so long as it does not add substantial grip for acceleration or braking.