Post cat 02s dont effect the mixture, They are there only to monitor the performance of the catalytic converters.
If you don't have a CEL, then i wouldn't worry about the o2 sensors until you are able to get a real-time reading to see if they are reading correctly.
Until then make sure the basics are in working order. Spark Plugs, Clean Air Filter, Fuel Filter, make sure there are no vacum leaks, maybe run some injector cleaner through the system and that the engine is warming up to normal operating temperature (a stuck t-stat will keep in engine in cost start mode and burn more fuel.
Also keep in mind that winter driving usually burns more fuel if you idle alot or let it sit to warm up.