Most ramp have rough surface to prevent people from slipping, it is the roughness of the ramp that slows down the movement of the toy car, so to allow the toy car to run fast put on top of the ramp something that covers the rough surface like a piece of plywood, another reason could be the elevation if the tilting of the ramp is very slight this also allows slow movement to make this move fats again place plywood on top of the ramp but this time at the top under the plywood put something to increase the tilting or elevation
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