With EPA ratings ranging from 18 to 21 mpg in the city, and 25 to 30 on the highway, the IS 250 and IS 350 models are about on par for the class, relative to other compact sport sedans like the Mercedes-Benz C-Class, BMW 3-Series, and Infiniti G37.
One thing worth noting if you're considering the IS 250: Automatic-transmission models of the IS 250 get significantly better mileage (30 versus 27) on the highway, due to a much taller top gear. It could make a big difference in the long run.
The one model in the lineup that isn't at all green is the high-performance IS F. It earns just 16 mpg city, 23 highway from its big V-8.
The 2011 Lexus IS family isn't that green, but it's about typical for this class of sport sedans; on the other hand, the IS F is a guzzler.