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I've been keeping track of the fuel my car, a Ford Fiesta Ecoboost 100 hp, uses since the day I got it. In the last three years I've owned it, I've noticed a large (>20%) difference in fuel consumption between driving in winter and driving in summer.
My driving patterns are identical in winter and summer, yet my fuel consumption is much higher in winter.
I understand that cars have slightly more power when cold, but is the difference that big?
Fuel consumption in liters/100 km:
Your fuel economy change is caused by the fact you're not using the same fuel. Fuel changes twice per year from summer blend to winter blend and back again causing a change in the Reid Vapor Pressure (RVP).
In summer, the hotter temperature evaporates liquids easier causing more pollution, so it is blended to lower the RVP. This blend is more costly to produce.
While in the winter time, the RVP can be much higher so additives such as butane are used. While this lowers the manufacturing cost, it also burns more quickly resulting in a hit on the MPG fuel economy.
Your vehicle does not operate at maximum efficiency until the engine has reached its normal operating temperature. This is generally when the coolant temperature reaches about 200 degrees Fahrenheit. In the winter it takes the engine longer to reach that temperature simply due to the colder ambient temperature.
Another reason: Winter tires. Choose between hard tires (good fuel economy, REALLY bad on snow) or soft tires (worse fuel economy, good on snow).
In the winter the ambient air is much colder, making the air more dense. Denser air will contain more oxygen and so the car will inject more fuel in order to maintain the correct ratios. This effect could be exaggerated by the fact your car is turbocharged and that the inter-cooler will be working much more efficiently in the winter, further reducing temperatures of the compressed air before it reaches the engine. This is also why your car feels more powerful.
When a car is started cold it will run in 'open loop' status. This simply means that the engine is running on a rich fuel mixture as the oxygen sensor won't work properly until it's heated up enough. When it is heated up the engine will run in 'closed loop' status meaning it will use the oxygen sensor to regulate the fuel ratio. During the winter it may take longer for the engine to reach 'closed loop' status. You'll notice the effect of this more if you make short journeys and not so much longer journeys.
I wouldn't expect either of these to have an increase in fuel consumption by 20% though, so it could be a combination of reasons.
Presumably in winter you'll have lots of additional systems switched on such as the heating, demisters, headlights, etc...
All of these things sap your fuel consumption.
Not sure if it applies to you, but when I switch to winter tires, I see a pretty radical drop in fuel efficiency, i.e., from 500+ km per tank to 400 km per tank of fuel, maximum.
Perhaps the air density may be a variable in this. Upon looking for a source on this, I found:
Finally, a vehicles aerodynamic drag is proportional to air density. On a 70-degree-F day, the density of the air is 16 percent lower than on a day with temperatures around 0 degrees F.
... in a Scientific American article.
I've got a 2015 Toyota Prius V, and with a fuel drop of almost 35% in the winter months. Could figure out the fuel lost until I did the research on different fuels. My fuel was just better than 51 miles per gallon, but left to see family up north mpg dropped to 29-35 mpg. Some of the problem was head winds going north and the following weekend again head winds. With a 20-30 MPH headwind and some gust as much as 40-45 mpg. Never though of a drop this much.
