We have the older version of the Prius...a 2002. Actual mileage varies due to outside temperature, but that's also assuming you are just watching the electronic information. One reason for the apparent "variance" is the fact that the fuel system is a pressurized bladder that expands and contracts not only because of the volume of the contents but because that volume changes with temperature. There is an adjustment that the dealer can do to get it closer to real, but it will never be accurate. The only way to really monitor your mileage is to do manual calculations each time you fill up.
The real problem with the mileage estimates on the sticker is the way they are determined...even the car manufacturers say the method is flawed, but they are required to post those numbers as the dynometer testing dicated by the government supposedly "evens the field".
That said, our mileage (mixed driving conditions) in summer is 49-52 mpg, depending if the A/C is running. Winter is 38-42 MPG, depending on how much the defroster needs to run. Dr. SWMBO's fuel costs have been cut in half, even during the colder months.
One other interesting thing about both the Toyota and Honda hybrids is the use of a continuously variable ratio transmission system. There are no "gear changes"...the transmission system adjusts the ratio to provide the best torque and performance based on what you are doing with your right foot, the speed you are going and other "immediate" conditions. Just as the engine shutting off all by itself takes some getting used to...so does the transmission! But it works well and when you stomp on the "gas"...the thing moves. (Our Prius can also turn on a dime, almost literally....I've never driven a vehicle with such a small turning radius...wonderful in parking lots and other tight spaces)
I would buy another hybrid in a second. Very pleased.
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The most expensive tool is the one you buy "cheaply" and often...