There's a belief that if the 'replica' is out by a certain percentage to the original then it will be ok - I don't know if there is any truth behind that.
If you are going for an exact replica of a current production car then you've got to expect a cease and desist to land on your door mat. If you are replicating an older car no longer in production then you may get away with it but as D-Angle says some like Ferrari don't like you doing ANY of their cars.
Basically if you are copying a design that is still under copyright then in the eyes of the law you can can be shut down at any time if the original manufacturer wants to do so.
For some reason that I don't particularly understand the original manufacturers more often than not just let the replicas manufacturers go about their business. A big chunk of the kit car industry would vanish overnight if the big guns decided not to play ball.
John
Madabout-Kitcars.com