If you're one of those people who religiously tallies your own fuel costs (using an app or a spreadsheet) you might notice that things take dip during the winter. With spring around the corner (at least for those of us in the northern hemisphere), warmer weather is on the way and at this point you might notice things improving in terms of the cost of running your car. With modern cars, loaded with electronics, comfort and convenience features, winter can 'cost' you up to 1mpg or about 0.75l/100km.
The reason for this is a combination of things.
If your car has heated seats, heated steering wheel, heated mirrors, heated washer jets or any combination of the above, and it's sufficiently cold, you'll be likely using one or more of those heaters each time you drive in the winter. The added electrical load means the alternator has to work more which puts a little extra load on the engine.
Then there's the fuel. During the winter months, many refineries distribute a 'winter' fuel mixture in an attempt to help with emissions during cold weather. This is actually a throwback to the 60's where emission technologies in cars were almost none-existent. Nowadays it makes almost no difference at all to the emissions, but the 'winter' blend typically has more ethanol in it and less actual petrol. Ethanol has less specific energy capacity so the more that it's blended with petrol, the less energy you get per volume in your car. In short, the higher the ethanol blend, the worse your gas-mileage becomes.
Finally there's the commuter stress. If you live close to where you work, during the winter months your engine likely never gets properly hot during the morning commute. Engine management systems tend to run a slightly richer fuel mixture in cold engines as well as altering the timing.
The combination of more electrical load, poorer fuel quality and colder engines are what add up to that 1mpg drop in economy during the winter.
On the plus side, the ice-cold air means the air charge going into your engine makes the engine a little more powerful (denser air means more fuel and air in the cylinder before detonation). You might feel this in the response and performance of the engine and that might encourage you to drive faster, which can also lead to lower economy :)