|MadSci Network: Physics|
It sounds as if part of your assignment is to choose the foods yourself, and to get them to work. Therefore, I'll limit my advice to general comments. 1. Try to mimic the real-world material properties with those of the foods. Properties you should consider are: 1) strength, 2) durability, 3) ductility (opposite of brittleness), 4) density, 5) cost. 2. A good substitute for rubber (tough=strong+ductile) is a high strength gelatin. You can reduce the recommended water content in unflavored gelatin for this. Gums (carrageenan, agar, etc.) work very well too. Another advantage is that you can cast parts. 3. A good substitute for steel/aluminum is low-moisture, starchy foods, such as pasta and dry breads and crackers. These are strong, but brittle. 4. Other useful materials are carrots, jerky and other durable materials. They are not very stiff, but they're tough. You didn't mention how long you wanted your car to last. This obviously would have a big impact on the types of foods used.
Try the links in the MadSci Library for more information on Physics.