Wheel slip is the critical question. If there is a chance for wheel slip, you must measure distance separate from the drive wheel rotation. If the surface is flat and smooth, just devise an accurate way to count the rollout of the wheels.
If you are concerned about wheel slip, put a tape measure on the back of the vehicle with the end attached to a weighted base. Then punch holes in the tape measure for absolute position points and measure the rotation of a pinch roller to add on to the last punched holes read.
If wheel slip isn't an issue, go for small wheels. That gives you better accuracy measuring a fraction of a wheel rotation. Slower vehicle, but more accurate.