The Model S should offer an option for a auto or manual multi-gear transmission not just one single gear. A five or six speed gear box might mean a more efficient and faster car.
A tranny might also allow Tesla to use a smaller electric motor since it wouldn’t have to turn such a large gear, this may improve range and efficiency. Auto makers like Detroit Electric and Brammo motorcycles have already incorporated six speed transmissions in there vehicles and Brammo explains why they don’t use a single speed gearbox https://www.youtube.com/watch?v=WzGaDIMlOy4
The Model S is a great car but if its gonna be competing with the big boys the single speed gearbox doesn’t seem the way to go. If you watch any race with the model S it usually wins when the other car is about to pass with it after they leave the starting line. This seems due to the one speed gearbox, it has extremely acceleration up to about 95mph and then falls flat. Also a transmission would reduce the rpm’s of the motor during highway drives therefore improving range
Editor: This would add weight, cost, and complexity to the existing system, and consume some luggage space.