Building and racing mousetrap cars is a common activity in many eighth- and ninth-grade physical science classrooms. However, once students have raced their cars, most mousetrap assignments come to an end. In this article, the authors developed a project to help teachers get more mileage out of mousetrap cars. The Mousetrap Car Project addresses the "National Science Education Standards" in motion and forces and transfer of energy. The project enables students to investigate the physics behind car construction and performance and is completed in four stages: assembly and materials, the race, data collection for the laboratory, and a final assignment that includes a to-scale diagram of the car and reflective essay. A logical extension activity might be for students to investigate the efficiency of an automobile and compare it to the efficiency of the mousetrap car. In this way, students may appreciate the difficulties involved in getting better fuel efficiency out of a full-size car. This culminating project is easily woven into the unit on simple machines or Newton's laws and can be used to explore, explain, and extend physics concepts that are traditionally addressed during this unit. (Contains 6 figures.)