Winter tires vs. MPG

I'm a relatively new prius owner. I just bought my '05 Prius in January. I have WS70 Blizzak winter tires on it right now (GREAT winter tire by the way), and I'm currently averaging 46-47 MPG in the winter cold of 0F to 30F on warm days. I've spent countless hours researching how to drive a prius and the best parts for it. I'm getting Michelin Energy Saver A/S tires in a few months in around April. My question is around how many miles per gallon do winter tires typically knock off? I'm very curious to see how many miles per gallon I will be getting with warmer weather and better tires.