You can not really drive a car to the next tile because there is no next tile. The endless level effect works with rendering imaginary images of the same level.

For endless terrain illusion, you must put back the car when it reaches a level border, in just the same way as the endless terrain script does it with the camera. The car is a physics object, so you must disable it before you can put it on a different position.

I havent tried it myself but this is the way it should normally work. Maybe you must overlap the tiles a little, otherwise the car could already fall over the border when its front wheels are outside the level border.