I installed the new carseat today. It just BARELY fits behind the passenger seat in our sedan. And then the front passenger's knees hit the glove compartment. And this is one of the shorter, cheaper carseats.
Obviously, once you have kids, it is only possible to get good gas mileage OR leg-room, but not both.
(We've been discussing getting a new car for 6 months, but figured we would hold off until we knew whether we wanted to get a new commuter car for nightskyre, or a bigger "family" car.)