@Number4 Not necessarily true. As the seasons change, temperature and pressure and humidity and all sorts of things change. Operating conditions are very different between the seasons. Pay attention to it over the summer and see if it declines over the winter; or look at your records if they go back far enough (you mentioned "historically"; have you been tracking it for several years?) and if you wrote the fill-up dates, you should be able to compare roughly what the temperature and weather might have been.
I noticed that my Elantra gets somewhere in the scope of 3 more MPG when the weather is dry, loses significant MPG when there is rain or standing water on the road, and loses about 2 MPG during the summer as opposed to the winter (averages 45+ winter, usually 43-ish in the summer, perhaps due to driving with windows down, not sure). Carrying passengers and using drive-throughs kills its MPG.
In any vehicle, carrying passengers/pets/groceries/anything will also affect MPG minimally on flat roads, but more extremely on inclines. Also take note of any tourist/beach/school traffic patterns, slow-moving traffic forces lower gears, which usually kills MPG. If traffic is diverted away from your region (such as a part of a community moving out of the mountains toward the beach during the warmer weather), that alone could net you a significant increase in MPG.
There are hundreds or even thousands of factors that you cannot possibly account for that could affect it, even if your driving habits and practices never change.
