After this many posts, and no one acknowledges that gearing and transmissions (10 speed for instance) can improve mileage. Some people mentioned technology, which can improve mileage as well. Even a good tune can improve mileage without taking off the emissions stuff. I know one reason for better mileage only through a tune is timing of fuel delivery. One simple example of this is my Duramax used to lug up hills in 6th, which sucks up fuel. If you manually downshift to increase the RPMs (assuming stock), it burns less fuel because it takes less fuel to push the same rpms. On the tune I have, it downshifts earlier without going into manual to save fuel.
So I reject the notion that huge car manufacturers with billions of dollars in R&D can't come up with more fuel efficient options.
