You also need to think about angle, cart coming toward you or away from you. At 1/160, a parallel shot of the cart at speed will have more motion than the same cart at the same speed coming toward you at an angle.

The shot below is at 1/160, which is a pretty high setting for any sort of panning shot. I chose this setting because I was trying to freeze the right-front tire off the ground (got close) while keeping some sort of motion in the tires.