Most car companies already have mature vehicle platforms (engines, transmissions, chassis and body experience, etc) to work off of and thus only have to charge R&D for a particular car against that car.

When you're developing a new car you have to do a lot of general R&D and a lot of car-specific R&D. What gets charged against the car and what are considered general? Who gets to decide? Why would they decide in a particular way?

It's not super obvious to me where you'd bill things even if they're ostensibly just for the Model S because in another year or two they might release a Model S2 with not a lot of R&D because they just stole parts from the Model S, Model X and Model 3.

Not saying it will happen, but it could. Then the R&D costs would be too low on that car and they'd have to go back and adjust their gross margin back as far as they've been selling the other cars and then the whole thing is a nightmare since I'm pretty sure the SEC won't let you do that.