Sounds to me like you need to put in a TX mix for the steering of the front wheels and the track speed. Never done this on a tank but in aircraft it's quite common to mix channels together to give you the correct flight characteristics like mixing together elevator and aileron on a delta wing airplane. or mixing in rudder with ailerons on a biplane to give the thing a correct and active turn.
Hope this is useful advice.
Cheers
CaptB