Once a car is built it needs to be tested to see if all the components are working together to accomplish the goals and meet the constraints placed on the design. Setting up measurable tests is an important part of the design process so that as adjustments are made to the car or its components the designer can determine if the change was beneficial or detrimental. If you find a major problem during testing you can return to the generate ideas step or look at some of the options you had during the compare and select step.

The testing phase is an appropriate time for the class to discuss performance measures. Some performance measures for the complete car include:

A long hallway or track and guide wires are useful for vehicle testing. Since solar power may not always be available for testing the vehicle, alternative power sources will be required. Teachers or mentors may need to help students mount batteries to their vehicles (note that car performance with batteries is unlikely to match that with the solar panel due to the internal resistance of the panel). A ramp may also be set up for "roll down" tests (motor must be disconnected from transmission) to test rolling resistance and aerodynamic drag.

Intermediate contests based on one or more of these attributes may be held to identify leading designs. The winners can be decided on the basis of relative performance of multiple cars.