Create a long and light "body" for the car.
The trap and the wheels are attached to this "stick". As seen in the pictures, the body can be smaller than the mousetrap. Trim it down; the lighter the frame the better! (but form boards break more often than boards)
When situating the trap, make sure the spring is facing the correct direction so the lever arm rotates forward.
Ensure the trap is as far forward as possible without touching the front wheels. The longer distance between the trap and the wheels, the better! (To a certain point.)
The wheels are a determining factor for a distance car. The front wheels don't matter in size or number; you can even have just one. As for the back wheels, you want the back wheels to be as large as possible, and the back axle to be as thin as possible. Old CD's work fairly well. A plumbing washer may be used to reduce the hole size in the middle of the CD (to fit the axle better).
To create traction, cover the wheels with tape, rubber bands, or balloons.
If they slip, energy is being wasted. Adding tape to the rear axle may reduce slippage of the string.
To attach the mousetrap to the frame, use glue instead of bolts.
The glue will hold just as well and the bolts just add weight! Make sure you have optimum placement of everything before gluing. A screw may give you a chance to change your mind, whereas glue is more or less permanent.
One possible vehicle.
The long body allowed a long lever (boom-box antenna), this increases the mechanical advantage by spreading out the spring's "snap" over a longer period of time. The energy of the spring is released slowly.
The mouse trap "killing bar" or snapper arm was cut near the far side of the spring and bent straight.
It was then inserted into the antenna. The trip switch and the latch bar parts were removed to reduce weight (they are not needed).
The string attached to the antenna tip winds around the drive wheel.
As the spring raises the antenna, the string spins the drive wheel and the car goes forward.
To improve distance, cut-out CD's were used for wheels.
They are larger and lighter than the rubber ones in the above images. In order to reduce mass and make them even lighter, you can take the extra effort to trim four triangles out of each CD. A small sink faucet type of washer can be used to reduce the center hole of the CD down to the size of your axle.
The car shown travels 6.5 meters on a smooth floor. Notice the fuzzy rodent next to the "cheese" which was added for character.