Whether you need a gyro also depends on top speed and how far apart your wheels are, the further apart the less likely you are going to need a gyro. My heavy, Blitz, had a top speed of 11.1mph, and 2 wheel drive with the wheels approximately in the middle and approx 61cm from the middle of one tread to the other. I seem to remember that the robot was fairly easy to drive on the smooth Robot Rumble Arena, but more of a handful on the more battle-worn Roaming Robots Arena. Basically, I think that if your robots got 2 wheel drive and a top speed of 12mph or more, a gyro is a very useful thing to have.