The concept car is a feature introduced with the Fortune Eggs. When you open higher level fortne eggs you have the chance of getting several special skins like tanks, robots, and pirate ships. To use these skins you must pick the concept car. These skins are the only things you can equip on the concept car. For a complete list of concept car items try looking at AutoTek, part of tektek.org. After you choose the concept car there the skins will be listed under the tab where paint normally is.
Link to AutoTek
http://www.tektek.org/auto/auto.phpHere are some examples:
General Urban Tank

Login Red-Driller

Pirate Ship Commander

Hope this answers your question.
~Avianflame
NSTG Crew