It may use a similar hybrid system to the Toyota Prius, but the sedan shape, more powerful engine, and considerable added weight of the 2012 Lexus HS 250h mean it doesn't achieve gas mileage anywhere near that of the ur-hybrid. The EPA rates the HS at 35 mpg city, 34 mpg highway, for a combined overall rating of 35 mpg. The Prius, which has more room inside--albeit far less luxury--delivers a combined 50 mpg.
And from our experience, that 35-mpg figure for the HS may be slightly optimistic--as those for hybrids driven to keep up with traffic sometimes are. In a fast-moving mix of Southern California traffic, our test car averaged just 30 mpg. In very gentle, carefully controlled conditions on quiet, level roads, though, it delivered 46 mpg. A different test car managed 32 mpg over about 100 miles of mixed traffic and road conditions.
And the 2012 HS 250h is no longer even the most economical Lexus, that title having been captured last year by the 40-mpg CT 200h compact hybrid hatchback which is, to boot, far more rewarding to drive.