Best outside temperature for optimal mileage?

[pedropedro]

New to the diesel world and I notice that when the outside temperature is below 40F that I see a several mpg drop. Is there an optimal outside temperature range? I would guess humidity would play a part too.

[chittychittybangbang]

Air conditions like the thicker air do lower mpg (combined with driving style (short/long trips)) But a much greater factor is the cold engine (takes longer to warm up and thick oil = resistance = parasitic power loss and lost energy) and winter fuel blend.

The winter fuel is cut with winterizing agents = less energy.