I would say that a car over $40,000 would qualify as a luxury car.
And. . .nothing wrong with the manufacturers building more efficient, smaller cars under the $40,000 level.
Yes, it would change the cost/demand curve, but with computer technology, it would take all of 1 week to get all the new curves in place.
Maybe it would even help convince people to buy smaller, more energy efficient cars. . .and, please, do not tell me they are more dangerous because if they are hit by a big car, they get more damaged!
The idea is to prevent accidents, not to built a tank to survive accidents! In fact, even if you take the biggest, strongest cars, they would be nothing in an accident with a truck. . .and you can't take the trucks off the roads!
Those "big cars" just give a false sense of safety. . .that's all!