Maybe you should check your thermostat as well as the O2 sensor you mentioned. A malfunctioning O2 sensor will certainly reduce gas mileage. Thermostats otday are designed to fail in the open position. This is good because the old way if your thermostat failed closed you would have no cooling at all and over heating is way worse than over cooling. It is also possible in cold weather that your engine temp will drop. I have noticed this, but mostly in sub-freezing weather, not much in balmy temperatures like you mentioned (we would kill for a day as high as 45F here). Especially when it is around 10F or lower the car will warm normally in city driving and get up to the 80 degree mark in just a little longer than normal. However, if you get on the freeway and get up to 60+mph all that 10Fdegree air rushing at your radiator drops the coolant temp pretty quick. You dash board temp indicator displays coolant temp only. What you might do before you test the thermostat is to see if there is a way you can partially cover your grill to keep this cold air out. This is done quite a lot up here, especially by truckers. They even have special custom padded grill covers. By keeping the cold air from the radiator you allow the coolant temp to rise much faster and be less affected by the outside temp. Of course make sure your air intake for the engine is not blocked, and most of these grill covers have some small flaps that can be opened (you have to get out of the car to do it) so that some air will get in. Blocking all the air from the radiator is bad as your temp will go up too much.
1995 E420 - retired
1986 420SEL - retired