The energy for a mouse-trap powered cars is stored in the mouse trap spring and it drives a wheel by some arrangement such as a string connected from the trap’s jaws. There was a competition among the mechanical engineers at my University to build a mouse trap car that could travel the furthest. There were plenty of different designs. The winners had a design that released energy from the spring slowly and had only two wheels. Thanks Stuartcw! Link.
How-To make Mouse-Trap cars