Cooler temperatures all by themselves can result in lower fuel mileage. It takes longer for the engine and its oil to warm up, as well as other fluids such as transmission and final drive oil. The fuel injection system may have to run a little richer until the cooler engine warms a bit, too.
If economy is your goal, you can do like I did and add a "short ram" intake. This would be the opposite of a "cold air" intake. Mine sits under the hood and pulls warm air from the engine compartment. It cost's me a couple of HP, but I have plenty of them to spare, so I welcome the additional fuel economy year round.
Yes. Winter blended gasoline has less energy per gallon than regular 'summer' gas. This is due to the fact that the blend has a higher vapor pressure (to aid starting in cold weather). The mix they use to get this higher vapor pressure (more volatile compounds) has less energy in it, thus we go fewer miles.
That's interesting, in Southern California I get less mileage from the summer blend than the winter blend, but I think that has to do with the increased oxygenator content for smog rules here. Back east you probably have fuel gelling issues due to the colder tempertures so there may be antifreeze type additives in the winter to prevent fuel ice and other issues that we don't have in less severe winters.
Before the switch to 10% mandatory ethanol in Southern California Gasoline, I could tell to the fill up when we switched from winter blend to summer blend by the drop in mileage that was around 10-20% on the same commute, same conditions, etc. No one has ever been able to explain to me how we get better air quality from burning significantly more fuel for the same miles traveled, but then again, I'm not an ethanol producer whose bought off our crooked politiicans and AQMD folks, so maybe that's what I'm missing.