Given its sporty demeanor and large V-8 and V-12 engine options, it's no surprise that the SL is thirsty. However, it's also lighter-weight and more advanced than past generations, which help keep the fuel economy from being completely abysmal.
The SL 550 is rated at 18 mpg city, and very decent 25 mpg on the highway, for a combined 20-mpg rating. Step up the more powerful SL 63 AMG model, and you only lose 2 mpg in the city, while highway ratings remain the same--lowering the combined rating to 19 mpg. If you're looking at the SL 65 AMG, however, you'll only earn 14 mpg in the city and 21 mpg on the highway, but even its combined 17 mpg isn't terrible–especially for its immense power.
It's slimmer, which helps handling, and the SL's also outfitted with stop/start technology, which shuts off the engine at longer pauses like stoplights, and automatically restarts when the accelerator is pressed. Some drivers may find it annoying, but it contributes to a 30-percent improvement in gas mileage on the U.S. cycle. The feature can be disabled via an Eco button on the console, but it must be switched off each time the car is started by the driver. The SL also has an Eco driving mode, one that slows out shifts noticeably and remaps fuel delivery.