I would guess that you are not pulling near 400 amps that the controller can handle. Especially when you figure that the voltage will drop probably around 30% once you "step on it".
Voltage pretty much determines speed, current multiplied by the voltage determines power. I kinda look at it like this (my general rule of thumb): Voltage has a direct relationship to speed, IF
, you have enough power to push the cart that fast. (not taking into consideration all the relatively
less effecting contributing factors there to which of).