I too am looking for snow tires. I have had my Prius for a little over a year and last winter my original tires did not do well. Acceleration was ok, but stopping was not good. Even in the rain I find that at times when I am trying to stop and hit a small bump the brakes seem to feel like they lock (the car feels like it slips for a moment).

As for gas milage in the winter, there are MANY reasons for lower gas milage including:

1. Yes, the combustion engine runs more to heat the block - usually to run the heater so the cooler you keep the interior the less it will run. This is the same for the air conditioner in the summer.

2. There is additional resistance on the road surface due to wet and snowy conditions. Especially with water, the surface tension and added resistance takes more energy to turn the wheels.

3. The gas is formulated different in the winter. There are more volatiles added (mostly butanes) that vaporize allowing the engine to start. Without this, there would not be enough vapors to cause the gas to ingite. This formulation reduces the amount of energy (Btu) per gallon of gas, thus causing a reduction in mpg.

If you have a Prius and have tried a certain brand of winter tires, please post what brand you had used and how well they work.