A mean looking line follower, do they have a 1/4mile dragstip event at RobotChallenge?
Nice idea!
My older son was on field robot event two weeks ago with his university team and (30kg) robot Beteigeuze:
http://fre2015.um.si/index.php/2-fre2015/14-beteigeuze
After the official contest they did a fun contest of "tractorpulling" with their robots 
Probably I would need to go with gearing as proposed (and calculated) by Mark.
I did fix the wheel with superglue, this is complete run (without accident/problems):
"Only" 11 rounds with 1.33m per round gives 5.63m/s or 20.26km/h, far less than with 48mm diameter wheel, perhaps because the 64mm wheel adds 2g of weight.
Perhaps less diameter wheels can give better results for the 12v motor -- with 6.9m/s maximal speed seen sofar the motor did show 2064rpm at the wheel although it could do 17500rpm.
Hermann.