What you are seeing doesn't surprise me. Snow tires are softer, much unlike the hard tires that come on the car. Mileage also goes down with temperature. In Eastern Washington my electric range on a full charge has dropped from 54 miles to 48 miles in the last couple weeks (with OEM tires still on), and like you, the temperatures are still fairly mild. I have bought a a set of studded tires mounted on rims ready to put on when the weather demands, and I fully expect another large drop in mileage when I do. I learned last winter that the OEM tires are horrible in snow and ice, so good winter tires and that drop mileage it just part of the reality of of owning a Volt. I don't plan on installing those tires until snow or freezing rain is in the forecast.