About 4 mpg in the Buick from the summer. I average ~27 mpg in the summer and 23 mpg in the winter. Snow tires, longer warmup times, and winter fuel all play hob with my winter fuel mileage.
The Fit is also down about 5 mpg for the same reasons. It's down from 34 mpg to 29 mpg.
We have junk fuel and junk, unplowed roads around me. All the localities do is put down salt now, they don't bother plowing until there's 2-3" of snow on the roads.