Most precise way to measure Linear distance travelled by a wheel
If you measure  one of the free running wheels (with no motor connect to it), it depends on the traction of the wheel, but that could be fairly accurate.
If you move horizontally a simple accellerometer (1 axis) would be great. But if you have a vertical component, then you can not distinct between accelleration and weight force (Einstein). If you use a 3 axes accellerometer, you can compute/extract the robots moving speed (subtract the G component = 9.81 m/s²) and thus the distance. You have to use vector arithmetic in this case.
If the ground has enough &quot;structure&quot; you could use an optical sensor (like optical mice have).
If you have a reference object (e.g. a wall, a book) you could use a LIDAR sensor (https://en.wikipedia.org/wiki/Lidar) to measure the absolute distances at time points and compute the relative distance between these time points.