I have a 2011 Prius I've had less than a year. This is my first winter with the car. I live in northern MN. I have put Nokia snow tires on. I have noticed that my gas mileage has gone from 51-53 mpg to 45-48 mpg. Is this typical? I am wondering if the snow tires are part of the reason, and if the engine runs more so the heater will produce for heat. Thanks.