I'm relatively new here (no prius yet) and I see a lot of talk of 17's and tire choice effecting MPG. The 3rd gen Prius with package 5 comes with 17's stock but it's still rated at the same MPG. Wouldn't toyota legally need to change that rating if it's a noticeable difference? Or is their tire choice pretty low rolling resistant? What tires come stock from the factory? What about the weight of the stock 17's? Thanks!