I've known it all along.
My wife has a 1998 LeSabre. By todays standards, it is almost a boat. She drives about 3.5 miles each way to work, and the thing manages an amazing 20 mpg. Set the cruise for 72mph and it gets 30. The thing has over 100,000 miles on it and the only major repair was the plastic intake melted. (This happened the week it turned 100,000 miles) I tore it apart thinking it was head gaskets and would you know, there were no ridges at the tops of the cylinders and you could still see the cross hatch on the cylinder walls. Car still runs great, even after I worked on it. When it dies we want to pick up a Caddy, but it keeps going strong, so that isn't happening anytime soon.
Most people think that if you want quality, you buy that foreign crap. GM has some great cars that get great mileage. They have great trucks with great power and great mileage. They just haven't seemed to want to tell anyone up until now. What the hell were they waiting for???