If you're using MPG as a basis for replacing a car, then you aren't replacing the car for the right reasons. First, you're buying a new car when you don't NEED a new car. Otherwise it would be clearly obvious which one to get rid of, and the MPG wouldn't matter. Which means the corporations (and not the environment) have already won.
Second, the pollution in creating a new vehicle, from melting and forming the steel, to the creation of synthetic materials in the cabin, to the electronics-- will probably cause more pollution than any MPG increase. For gas-electric hybrids, the batteries alone create a significant negative environmental impact. Which means that while your personal actual output will go down, the results of your purchase causes pollution to go up. Which means the corporations (and not the environment) have already won.
Personal carbon emissions are really just a drop in the bucket of the worldwide problem. Even gaining 1000% efficiency when driving won't swing global warming either way as long as we're willing to dispose of perfectly good cars simply because we want to get to pick a new one.