The 2014 Mercedes-Benz GLK does much better on fuel economy than do earlier models of the same vehicle built between 2010-2012.
With the V-6 gasoline engine, the GLK 350 with rear- or all-wheel drive is mostly rated at 21 mpg combined, with slightly variations from 18 to 19 mpg city, and 25 mpg highway). The one exception is the slightly cleaner "PZEV" version of the rear-wheel-drive model, sold in about a dozen states that use California's stricter emissions standards, which comes in at 22 mpg combined (19 mpg city, 26 mpg highway).
Opt for the torquey, fuel-sipping GLK 250 BlueTEC with its turbodiesel four-cylinder and 4MATIC all-wheel drive, and your numbers improve considerably. It's rated at 28 mpg combined (24 mpg city, 33 mpg highway).