The C250 sedan scores 21 mpg city and 31 mpg highway for 25 mpg combined, according to the EPA. The C250 coupe matches the sedan's figures.
The C300 4MATIC, available only as a sedan with all-wheel drive, rates 18/25 mpg for 20 mpg combined. The C350 sedan fares a little better, scoring nearly as well as the C250, at 20 mpg city and 29 mpg highway for 23 mpg combined. The C350 coupe scores slightly worse, at 19/28 mpg and 22 mpg combined.
As the most powerful C Class model--by a significant margin--the C63 AMG is also, predictably, the least efficient. It scores an estimated 13/19 mpg for 15 mpg combined according to the EPA.