I live in Wisconsin and just finished a trip between Lacrosse to GreenBay. Outside temperature was -8F (....) I was driving from 4:30pm - 7:30pm. I filled up my car in Lacrosse and started with a full tank.

My tank average is 33.5MPG, with
Hwy 90 - 70mph
Hwy 21 - 63mph (when I was passing through town, the speed is limited to 25 and i was using electric motor only, but it is only less 10% of the trip)
Hwy 41 - 70mph

Usually i can get up to 43MPG when the temperature is over 32F