Just wondered if it could be air density helping the combustion along in the winter but messing it up by being . . less dense in the winter?
Air is denser when cold.
Note that the denser air has the ability to hold more fuel that lighter air. This may appear counterintuative, but airflow to an engine is virtually constant (except when the engine is warming up on choke) Warming up the engine, simply aids combustion.
Given the average, there is only so much that can be managed, given the numerous single points that are common. ie, the computer will try to balance the OAT, intake velocity, intake volume with the fuel source. All fuel is not equal, so IF your vehicle has a fuel sensor, it can optimize that curve, if it does not, it will relegate the same average metering.
Average fuel metering, winter vs summer, creates a significant difference in economy.
As you know from aviation, there may be different additives, depending on weather, but also in general.
Diesel is especially notorious for a plethora of additives, meant to 'cut' the volume, the temp range, or just to get rid of PCB oil.
Do you get D1 or D2?
D1 is a thinner blend, and you wont get as good a mileage
Diesel can be cut with 5 to 10% biodiesel as well, this will actually increase capacity in diesel...
D2 with 10% bio is a monster of a blend!
