"The 2010 Prius debuted today in Detroit and, well, it's a new Prius. But it's also a little bit more in a lot of little ways. First, if all you care about is mileage, the number you need to know is a combined rating of 50 mpg, based on Toyota's own internal testing. That bump in MPGs (up from 46 in the current model) comes from a larger and more powerful 1.8-liter Atkinson-cycle, four-cylinder engine. That may sound counterintuitive, but Toyota's press release (pasted after the jump) is prepared for that, and says that the higher torque means lower rpm on the highway. And the highway is where hybrids don't exactly shine." (2009, Autobloggreen.com)