This clipping was taken from MSNBC.com

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EVERY CHANCE I could, I'd look at the display screen that tracks mileage, pleased when it showed 70 mpg or no gasoline being used at all because it was running solely off the battery. But over the entire 200 miles I drove the car, the average picture was not as pleasant and I never got close to the 51 mpg highway and 60 mpg city certified by the Environmental Protection Agency. Instead, I averaged 42 mpg in combined city/highway driving.
Frustrated, I sought answers from Toyota and its Prius guru, Dave Hermance. As soon as I said that my commute and other city driving were short trips, Hermance interrupted. "That's it right there, that's the problem," he said, explaining that mileage suffers during the first five minutes of a cold start because of the way the Prius and similar low-polluting cars curb emissions. That would also explain that while my combined city/highway mileage was 42 mpg, I got just 31 mpg on my five-mile commute to work. The mystery solved, I still felt some disappointment that I'd never see 60 mpg with my short commutes. Instead, as Hermance noted, "it's an ideal car for folks who put on a lot of miles." On the other hand, that overall 42 mpg was still double what my 1990 Nissan Stanza gets - and a lot less polluting. The 2004 Prius emissions are 90 percent less than a conventional car, and 30 percent less than the 2003 Prius. Emissions of carbon dioxide, a gas many scientists fear is warming Earth, are about half those of traditional cars.