Posted average fuel economies
Posted average fuel economies are great when choosing a car, but I think it only matters when you're choosing between 6 cylinder turbo and a 4 cylinder 1.5 L. That's where you will see significant savings.
No matter how accurate the average fuel economy might be, I think the most important thing is to do what you can to keep it as low as possible on each trip. Drive slower, accelerate more smoothly, combine your errands into as few trips as possible, drive less. This is where you'll find significant savings rather than nitpicking over average MPG accuracy here and there. My RAV4 hybrid can have amazing fuel economy if I just drive better. When I'm late for an appointment and time is of the essence, my fuel economy sucks.
In summary.... pick a car with smaller engine (hybrid even better, PHEV perhaps best of all) and drive in a more efficient manner, or drive less.
From a fuel nerd perspective, what I'd really like to see on my RAV4 hybrid is a report on the EXACT amount of fuel I burned per trip, rather than averages. Does anyone know if there is a plug-in diagnostic tool that could report on actual fuel consumed?