https://www.greenhybrid.com/compare/...oltswagon.html

Check out Voltswagon. A couple years of data have been input for this car so the graph is nice and averaged out. Notice that the highest mileage comes at June and September because those months are warm but not so hot as to require a lot of AC. Mileage is down a bit in July and August because of AC use. Even with AC use in mid summer the mileage is still higher than in the cold months.

 

i actually see my mpg improve on warmer rather than cold days, and i almost never use a/c (once in a while only to keep the compressor and system working properly)

my 2 cents are, another possible factor would be that increased temperature leads to increased pressure inside the tires, which helps to increase mileage.

(pressure is directly proprotional to temperature for gases given the same volume and moles of gas, PV=nRT where P is in atm, V is in Liters, n is in moles, R is 0.08206 L*atm/K*mol, and T is in Kelvin, also known as the ideal gas law)