The 2012 Volvo S80 can be reasonably green--if you choose the right model. The all-wheel drive version of the S80 hits pretty hard--2 mpg in the city, and 3 mpg on the highway, because of the additional weight and drag even when you don't need the system.
But with the six-cylinder engine and front-wheel drive, the S80 can be a modestly green pick--if you plan to use its capacity for carrying adults. Volvo has for this year managed to boost its ratings for the front-wheel-drive S80, by 1 mpg in the city and 2 on the highway, to 20 mpg city, 29 highway, and that's now impressive for such a large sedan.
Although we haven't driven an S80 in some time, in the past we've seen numbers toward the low end of the range, in mixed driving. At more than 4,000 pounds, it's quite heavy, and that likely has something to do with it.