Use plates with slide above and below beams to make a gear train with the driver gear on the motor in the middle and two 40 tooth gears for the wheels.
An 8 tooth or a 24 tooth gear can be used on the motor.  The length of beams can be longer or shorter depending on what else you want to attach, i.e. touch sensors.

Link to line a follower page
from http://netapps.marshall.edu/lego/lessonplans/Houston/legoprojects.html