We haven't found the Turbo6 engine, as it's badged, to be very fuel-efficient in real-world driving, either. Over about 150 miles of driving, split over a couple of days—mostly a mix of crawling along in Washington, D.C.–area traffic and commute-style conditions on some of the surrounding freeways and countryside—we only managed to average about 18 mpg.
With respect to fuel consumption and carbon footprint, the Saab 9-5 is about mid-pack among larger luxury sedans in V-6 form. But with its base 2.0T four-cylinder engine, the 9-5 is actually quite fuel-efficient. EPA figures of up to 20 mpg city, 33 highway make it one of the best in this class—if you're okay with front-wheel drive. As soon as you opt for one of Saab's excellent XWD (all-wheel drive) alternatives, figures drop—to 18/28 with the four-cylinder, or a less impressive 17/27 with the turbo V-6.