My winter MPG losses at last winter were because of winter diesel, because engine could not reach optimum temperature in cold, I needed cabin heater which took so much heat that even with grill blocks it was too cold wind under the hood for engine to stay warm, also because most of driving was in deep snow, sometimes more than 4 inches and they are now going to reduce road upkeep further...
It might be even worse with warmer air, cold air probably helped a bit as cold air has more oxygen.
Boost gauge did sit at 0.2-0.3bar when driving on snow, while normally it is at 0bar mark, maximum is around 1bar.
With petrol car that had been given bit more aggressive cam etc. it was interesting to note that during the summer at cold places (temperature sensor at intake pipe and laptop logging / instruments), car did go easier, throttle position must be lowered to not increase speed, injector open time was then shorter.
Of course there must be the limit for that, if we think about ethanol, it has trouble making good mixture with cold air, which partly leads poor starting below zero, maybe there are such points for diesel too, glow plugs are there because of reason, if colder would always be better surely glow plugs would not be needed.
Compression makes diesel and air hot, so that it does ignite, if air is so cold that compression does not reach enough heat, then igniting will be poor, so surely there is optimal temperature and too cold will not be good.
Some effects are of course because of cold fuel, which makes it bit harder to find out which is what.
I vote for 10C or around there, purely a guess, but I would consider that being good one.
Then think about humidity in air and how water increases efficiency, in cold air there is less water, what that might affect? I know that after the rain when weather is damp car likes to fly
