What is your basis for this belief? Just curious. It's an interesting topic but is there any supporting evidence for thinking Apple may make a car by 2018?
To answer your topic question, I don't think it would make a huge impact from strictly an association standpoint. People associate Tesla with electric CARS. People associate Apple with electronic DEVICES (Phones, Laptops, iPads, etc.)
A while back, a LONG while back, I did a focus group regarding an "Apple Airline". They had a mock-up plane, colored the trademark Apple white, with all sorts of bells & whistles and fancy tech inside. The whole group, myself included, couldn't support it. Everyone felt that even if it was "geared toward our demographic", that Apple had no business making airplanes. Funny enough, it just seemed to be the group decision and there was no swaying us.
If I were to do a focus group about an Apple CAR today, I would feel the same way. It's not their niche, it's not their business, and it would feel hokey and almost silly to even say you "drive an Apple". Gimmicks like color options reflecting their products, iPad dash, iTunes built-in, all that stuff still wouldn't make me feel like I was making a wise car-buying choice. I would feel more like "my car is going to outdated in 6-months".
I may be alone in this opinion, but I don't think an "Apple iCar" would pose any significant threat to the model ≡ market.