Test the strength of tires by testing different types of tires on different surfaces. This works on several different levels, depending on your budget and the interests of the student. Place a set of tires on an RC car and run it across a paved surface. Then take pictures and document the tire wear, before switching to a different set of tires. Older students may want to test tire wear on different types of vehicles, by noting the way tires wear on two vehicles across the same stretch of road.
Researching the level of car accidents in different areas is a science fair project more dependent on research than experimentation. This is a good project if you have a specific area with a high level of car accidents, such as a certain intersection in your town. Investigate when the accidents took place and the weather conditions. Let the child draw his own conclusions based on his research, including what conditions are worst for accidents.
Research major changes in the history of the automobile industry to show how car safety has changed since the first cars were introduced. Make note of any major changes to a specific type of safety device, such as how the first seat belts were significantly different than today's seat belts. Then, draw some conclusions about the future of car safety and what manufacturers should do in the past. Focusing on car safety in NASCAR is an idea for students interested in car racing.
Focus the science fair project on hybrid cars. Younger students can focus on exactly what a hybrid is and what companies make those cars, while older students may want to focus on how a hybrid works. Explain the difference between hybrid cars and cars that run on fuel, along with why hybrids are viewed as the future of the automobile industry.