GM has one of the most fuel-efficient full-size pickup trucks on its hands with the 2012 Sierra, but it also has some of the least efficient versions, though its V-8s aren't always the culprits.
The GMC Sierra Hybrid, as you might expect, earns the best EPA gas mileage ratings, at 20/23 mpg. Just behind that, however, is the 5.3-liter V-8 edition, which has cylinder deactivation technology and six speeds in its transmission, plus an XFE model with aerodynamic add-ons. It's rated as high 15/22 mpg--better than the Sierra with the base 4.3-liter V-6 and its 14/20 mpg. The difference in large part is the transmission, since the six-cylinder truck has an outdated four-speed automatic.
There's a fleet-duty 4.8-liter V-8 rated at 14/19 mpg, too. The Sierra's luxury-edition V-8 cuts its best figures by at least a third, rated at 13/18 mpg.
GM doesn't offer a diesel on the light-duty Sierra.