Mark Rober is an ex-NASA engineer and is extremely smart. He makes a number of helpful videos and posts them on YouTube. This time around, he shows how to build an efficient mousetrap car. He explains that the designs should be such that it delivers the smallest force over the longest distance.
He then explains that friction needs to be reduced. This needs to take place where the tires meet the ground and the axle meets the body. Finally, you need to reduce the weight of the car. You can learn how to do all that in the informative video below: