[Alan Anderson]

Driving a six-wheel skid-steer drivebase using Tank Drive (left and right joysticks) is not easy if the robot doesn't want to go straight, even with computer control based on a gyro sensor. It is very easy to make it go straight using Tank Drive if the robot is properly constructed.

But if you really want to try to compensate for a difficult-to-control robot, the idea is simple. Compute the difference between desired and measured heading, scale it to something that will yield a value between -1 and 1, then add it to the joystick input for one side and subtract it from the joystick input for the other side.