I have heard very dramatic claims about Prius mileages. Some peope are only getting around 30 MPG's, while others are getting even above 60 MPG's. I think it all depends on how one drives, and what type of conditions they drive in.
I have rode and driven around many people in conventional cars, and have noticed that some people drive very roughly and abruptly with their vehicles. They accelerate very quickly from standing starts, even when there is no apparent need to. When they need to come to a stop, they wait until the last minute, and brake really hard. Frequently, I see drivers not really anticipating traffic very much either, and will accellerate hard and fast to the next block, where the next red signa is awaiting. Tailgating other vehicles is also very wasteful, as it takes constant accellerating and braking to match the speed of the vehicle ahead, because there is no buffer zone. All of these acts not ony waste gas in conventional vehicles, but completely hinder a hybrid system from doing what it does best.
Hard braking activates the hydraulic brakes to pick up what the regenerative system cannot do, and the momentium energy is lost. Fast acceleration means that both the electric system, and the gasoline engine are providing power, which is a less efficient mode. I've seen many Prius drivers that are smart though, and keep the vehicle under 30mph between traffic signals, which minimizes use of the gasoline engine. I imagine those that are complaining about dissapointing fuel mileage in the Prius are ones that are driving the vehicles very poorly, versus any type of electronic or mechanical problem! Luckily, most of the prius drivers I see out on the road keep a nice steady pace, and aren't driving like they are in a NASCAR race.
One last thing that can reduce a few MPGs is the use of the air conditioning in the climate controls system. It is an all electric system, so the gasoline engine may need to run more frequently to recharge the traction battery when you have the AC on. It probably reduces MPG's more in town than on the freeway though. At freeway speeds, the excellent streamlining would be broken up by the open windows. I imagine though wintertime and summertime fuel mileages are a little different when used on the same road & conditions (where the windows are up, and the AC is not on)
Still, hybrid technology can only go but so far in making a vehicle more efficient. Heavier vehicles will have poor in-town mileage because more energy is needed to overcome inertia, and make the vehice move from a standstill. Aerodynamics, and wind resistance plays a big role of fuel economy at highways speeds. SUV's typically get knocked with both shortcomings. They are both heavy, and have very poor streamlining. Streamlining would take away from their "rugged" look, and their raised chassis create considerable wind resistance underneath. The Ford escape is the perfect example of this. It is a full hybrid just like a Prius (and in fact Ford buys Toyota's technology to use in it!). It can only eke out 27 MPG's. It is considerably heavier and boxier than a Prius. Lucklily, many car manufacturers are now starting to build "crossover" vehicles...IE Station wagons again. For example, the Dodge Magnum wagon, a full-size vehicle, gets 27 MPG's on the highway, but is not a hybrid. It's long, low body helps it achieve low wind resistance. Ford also has out a "freestyle" station wagon based on the 500 sedan that also achieves good mileage
