Like most large SUVs--but unlike Ford's own F-150 pickup--the 2012 Expedition isn't very efficient with gas, or very green.
The EPA rates the Expedition at 14/20 mpg, when it's equipped with rear-wheel drive. With four-wheel drive, the numbers fall to 13/18 mpg.
In contrast, the F-150 pickup line--from which the Expedition is derived--now has a new lineup of V-8 and V-6 powerplants that have boosted its fuel economy considerably. With the likes of the EcoBoost V-6, the F-150 now gets highway ratings solidly in the mid-20-mpg range.
Ford hasn't confirmed it, but the Expedition is due to receive at least some of these improvements at its next major model change.
That said, much of the Expedition's efficiency depends on use. If you use all three rows most of the time, and park it when it's just going to be the driver, the Expedition can be a reasonably green vehicle.
Also, for those more interested in petroleum independence than environmental impact, these big trucks are E85 (85-percent ethanol, 15-percent gasoline) compatible, but then consumption numbers fall to an EPA-estimated 9/13 mpg.