I found this to be rather weird. F150 5.4 Lariat 4WD, although old I only have about 137K on the odometer and recently had to replace the alternator (nothing else, just the alternator). Gas mileage on my run to hunting camp and back has been ~ 17.6 to 18.0 depending on load, consistently for the last 15 years. With the new alternator I'm now getting ~ 18.5 to 18.75!

Figure that one out.

 

Change in fuel quality... change in weather... other than the old alternator dragging and causing a parasitic loss Im out of ideas

 

It's finally broken in!!!

 

I agree with Jay, winter fuel usually uses more ethanol, and different gas companies have different additives they add to their fuel, so if you switched, it could go up or down.