Cybersled

What prevents the design of a super fast, lightweight tank?  It would rely on speed and manueverability, lack armor, and use rocket propelled grenades as weaponry.  It would rely on thrusters and propulsion instead of the standard tank track.  Could such as design exist and still have reliable fuel efficiency? 

>>4
These also exist. They are more often employed by national guard or big city swat forces. Problem with wheels is that wheels mean tires, and tires me flat tires.