I think the only way to have a girl understand cars is to find a hot doctor/nurse. Give the car eye lashes and paint the engine parts different girly colors, maybe throw some sparkles in there, set some red high heels right next to your wheels, aaand put a glamorous wig over the dash above the cluster.
Then explain how cars are like humans and point out how gay you just made your car look. But because she secretly wants a gay best friend, she'll totally understand everything you say about your car.