MORE ECONOMICAL S.U.V.s on the way: “However, despite its full size stature, room for seven humans and a few cold drinks, the Flex doesn’t attack your gas budget like Cookie Monster on a binge. Its 3.5L V6 engine gets a respectable 24 mpg on the highway, assisted by a high-tech six-speed automatic transmission. Down the road, when the Flex is available with a turbocharged direct injection Ecoboost option, its fuel economy should raise a bit more.” You may not think that’s a lot, but if every SUV in the country got 24 mpg on the highway, the difference would be huge.