Hi,
Just for the record, I swapped over to my snows (Michelin X-Ice) and I installed smooth wheels covers at ~1/2 tank -- and my average for the whole tank jumped up ~2.76mpg over the previous tank!
I'm luvin' these wheel covers! I'll see how this current tank goes...and if it's proportionally better, then I'm definitely keeping these rims on the car next spring! (Do you know anybody who wants to buy some 15" alloy rims?)