I have a 2011 Prius, and have gone thru my first winter. Last Nov. when I put on my snow tires, I noticed my gas mileage went from 51+ to around 45. I read on here that that was normal because of getting the engine heated, the enterior heater, winter gas formula, etc. I just switched from my 4 Nokia winter tires back to my original tires, and the mileage immediately jumped back to 50+. It is still relatively cold here ( northern MN. -8 this morning). Can winter tires alone cause that much of a difference? Maybe the "knobby" winter tires with soft rubber just don't roll that easily? Anyone else notice this?