Anyone who uses GPS tracking a lot will know the situation.
Sometimes it overestimates and sometimes it underestimates (in real life, not in the fake conditions these guys set up).
GPS traces often show corners being cut because position is only recorded every few seconds and distance recorded based on straight-lines between those points.
At other times you get a weak fix and find that you've been weaving all over the road (and adjacent fields, rivers etc) thus appearing to be further than you really travelled.
Heck, I've even hit 18,500mph while cycling - apparently!
Biting the hand that feeds IT
