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When I have these pop up mystery problems I normally start by clearing the cars computer by disconnecting the battery for a while. Then I look at maintenance due, like miles since last service, air filter, spark plugs, O2 sensors, etc. Also I run 50 psi in my tires which is 15 over the side wall rating.
But my old van always got worse mileage in winter than summer from the temps, and my winter tires, etc. I was consistently getting 27 - 28 mpg on the highway in the summer. Verses 16 - 20 mpg in the winter depending on the outside temp. The winter engine warm up time was also a contributor to my mileage loss.
These are the obvious things to start with. Then others will likely chime in with more specific suggestions.
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