Part of the kids' learning would be to note that eventually the same on-time gives too short a distance and they would chat about that and conclude it's as the battery goes flat.... then that could lead to a whole new chat and experimentation on different types and brands of batteries, looking into how to overcome the battery problem, solar power and blah and blah. They will, off their own bat, I'm pretty sure experiment on up- and down-slopes, different surfaces like pile carpet.
Pretty soon the 2013 equivalent of a 1970's Mopar fan will have the Arduino equivalent of a 426 Keith Black Hemi in there.
This is an awesome piece of learning... kudos to the G&T program.