A couple of things I wanted to bring up that I don't have the math to back up:
First: How many people can afford a more expensive and profitable bottom tier SUV if they were looking at an inexpensive small car?
Would the price increase to a more profitable, less efficient style lead to a lower total number or automobiles bought such that the total fuel consumption is lower than it would be if there were more affordable options that happened to be more efficient? Maybe a family having one SUV instead of two small cars or someone who has to stretch too far to get a small car, and decides against a vehicle at the SUB starting price.
Second: To encourage the use of public transportation be it buses or subways when they are associated with the dirty, slow, last resort options of the lower classes. Would it make sense to have premium buses and designate one or two cars on the train as premium cars that cost more, are more luxurious, and attract a more affluent crowd? They have to be in aggregate more efficient than personal cars, but cool enough to attract the right people. They also have to be expensive enough to discourage the lower classes from using them but not so expensive they aren't used enough to be profitable.