94 E320, winter MPG down 5?

[wolf_walker]

Took a 5mpg hit the last two tanks, car runs normal, girlfriends work car.
Started about time temps really got winter like here (oklahoma), premium non-ethanol always same station. Her commute is kinda short and unfortunately she has to run two kids in two separate trips each morning a quarter mile to the bus stop and back, then leave for work an hour later. Sort of a worst case scenario but such is life. Should I be concerned or just the nature of the season? My first winter with it, gone from 22avg to 17.