Sol, BOTH WillWren and compyelc4 are correct. You are due for a new O2 sensor (as am I), that is a portion of your bad fuel economy. But compyelc4 has a valid point too, although he failed to mention in greater detail what the snow does to mileage. Snow and ice cause you to drive much slower--sometimes around 35 mph. This is not the optimal speed for fuel efficiency in any condition, usually our Bonnevilles do better around 50-60 mph. Add to that the additional friction of driving on snow or slush (have you ever noticed how much harder it is to walk in sand at Lake Huron, Lake Michigan, or any of the other Great Lakes? Or how much harder it is to walk through deep snow? It is the same principle). Loss of traction is also a factor. Whether you want to or not, you will break traction on ice, snow, and slush. There is also a colder, denser air charge going through your intake plenum which requires more fuel to maintain a proper fuel to air ratio.
To sum up, winter means more power, less traction, and less fuel efficiency.
Change your O2 sensor but don't count on the same mileage you get in the spring/summer/fall.
Retired Moderator, The Bonneville Club www.bonnevilleclub.com
RIP Oldsmobile, April 29
1995 Bonneville SE Supercharged (Stock)
Current Mods: Dirty GYM Sock air freshener.