My "Fitbit Charge" would count *thousands* of steps when I was driving my Landie (Defender 90), especially if I was off-road. I guess the vibration was just right. or just wrong. I never got much of a count driving a "regular" car however.
This weekend was the first time I've been out off-road in the 90 with my GearFit 2 on, and it didn't register any significant steps over what I know I actually did, so I guess it's much more accurate in this regard than teh fitbit was.
So I wonder if yours is over-sensitive, or if perhaps your particular car jut happens to generate movements that set it off. Does it matter which car? What sort of roads?