The 2014 Lexus IS gets fair-to-good marks for its gas mileage as rated by the EPA, considering its performance and luxury class, though it's certainly no hybrid rival.
The 2014 Lexus IS 250 sedan rates 21 mpg city, 30 mpg highway, and 24 mpg combined according to the EPA; with all-wheel-drive, the IS 250 AWD drops slightly to 20/27/23 mpg city/highway/combined. The IS 250 C convertible rates the same as the sedan, at 21/30/24 mpg.
Lexus' more powerful IS 350 models are only slightly less efficient, earning gas mileage ratings of 19/28/22 mpg (IS 350); 19/26/21 mpg (IS 350 AWD); and 19/27/22 mpg (IS 350 C).
In other words, there isn't a very big penalty for choosing all-wheel drive with either engine, but the 2014 Lexus IS is not especially green in any form. Gas misers will do well to look elsewhere, particularly toward turbocharged four-cylinder alternatives in the compact luxury sedan segment.