Ever since they started publishing EPA mileage ratings on new vehicle window stickers, those stickers have also carried the disclaimer that your mileage may vary. The official ratings are based on a standardized test sequence that all light duty vehicles are put through. In order to compare different vehicles you need to test them in the same way. The problem is that in the real world, not all drivers behave in the same way, and few, if any, duplicate the type of sequence modeled by the tests. The result is that almost no one matches the published numbers. The newly-revised procedures for 2008 get closer to what people can expect in the real world but variation still exists.
Hybrid vehicles are even more susceptible to driver behavior and operating conditions than conventional vehicles. Driven with a bit of a lead foot or mainly on the highway, hybrids often fall far short of expectations (just ask John True). On the other hand some behavioral changes can see you using far less gas than even optimistic sticker ratings. The latest post on the Toyota Open Road blog gives some tips on hypermiling that can be applied by drivers of any vehicle, whether in a hybrid or not. Obviously manually turning off the engine instead of idling doesn't apply to hybrids since they already do that. The other tips can help all drivers and most of them shouldn't be too obtrusive in everyday driving. If you haven't already tried them out, now is a good time to start.
[Source: Toyota Open Road blog]