Does anyone happen to know when winter formulation fuel is phased out? I usually see a drop around the first fill up in October.

I am guessing that the end of March / beginning of April is range when it is phased out, mainly because I have seen a VERY SIGNIFICANT increase in mileage this last tank - I am talking like 10% better FE - and I do not believe it is the slightly milder temps...

Anyone else in the Midwest notice a big difference?

Yes I do notice a drop. April 1st is the time that everything changes over. That is why you sometimes see an overnight jump of 15 to 20 cent. I live in Kansas City and I have noticed a drop but usually that is offset as the weather warms up and the car has less and less deep cold starts.