As if we really needed another reason to hate winter.

Those of us living in northern climes have already started to notice the seasonal decline in fuel economy, even with careful attention to sagging tire pressure (probably the best known effect of the mercury's slide).

Yet, despite diligent all-around maintenance and continued careful driving, cold weather fuel consumption can be dramatically worse than in warm temperatures.

How much worse?

Have a gander at these calculations for a hybrid at 60 MPH in varying ambient temperatures:

MPH-----AMBIENT-TEMP-----MPG

60------------95----------52.98
60------------85----------52.62
60------------75----------51.16
60------------65----------49.12
60------------55----------47.22
60------------45----------44.67
60------------35----------43.05
60------------25----------41.54
60------------15----------39.41
60------------05----------38.09

Look at the extremes: the coldest MPG is 28% lower than the warmest. (Source.)

Why so bad? Here are some reasons why:

9 reasons your winter fuel economy bites

1. More idling
   This should be a no-brainer, yet parked idling cars are a common sight in cold weather. Resist the temptation to idle your car to warm it up. An idling engine gets 0 mpg. Consider also that an idling engine does nothing to warm up the tires and drivetrain.Even in the coldest weather, you can begin driving after 30 seconds from a cold start - keep speeds low/moderate and use gentle acceleration until the temperature gauge starts to climb (source).
2. Low tire pressure
   Of course you're smart enough to keep up your tire pressure as the temperature drops, right? A 10-degree (F) change in ambient temperature equates to a 1 psi change in tire pressure (source). Fuel economy declines 0.4 percent for every 1 psi drop (source).
3. Increased rolling resistance
   Even if you're completely attentive to proper tire pressure, cold ambient temperatures will still cause your tires to return worse mileage. That's because a tire's shape isn't completely round - the sidewall bulges out at the bottom, and where the tread meets the road the small contact patch is actually flat. As the tire rotates, it constantly deforms to this shape, and this deformation requires more energy when the rubber is cold and hard. Rolling resistance at 0 degrees F is 20% greater than at 80 degrees (source 1, source 2).
4. Crappy road conditions
   It is increased rolling resistance of another kind: driving through slush and snow. And then there's its wasteful polar (no pun intended) opposite: no friction at all! (A.K.A. wheelspin on ice.)
5. Lower average engine temperature
   In the winter, an engine takes longer to reach operating temperature and cools off faster when shut off. Since the engine management system orders up a richer mixture when cold (proportionately more fuel in the air/fuel combination), more fuel is being burned overall.
   
   A block heater can offset this problem (improving fuel economy by 10% in sub-zero conditions - source), as can garage parking, and combining trips (to minimize the number of cold/hot cycles).


6. Also related...
7. Higher average lubricant viscosity
   Engine oil thickens as it cools. So does transmission and differential fluids and even bearing grease. Significantly more energy is needed to overcome the added drag these cold lubricants cause.
   
   Using synthetic fluids can address this problem, since their viscosity changes less at extreme temperatures than traditional mineral fluids.
8. Weaker gasoline
   Gasoline doesn't vaporize readily at very cold temperatures. So oil companies formulate fuel differently for cold-weather markets in the winter. Unfortunately, the changes that provide better cold vaporization characteristics also result in less available energy for combustion. You won't get as far on a liter of winter gas as you will on a liter of summer gas. (Source.)
9. Higher electrical loads
   In colder temps, you use electrical accessories more often:
   1. lights (in higher attitudes it's darker in the winter)
   2. rear window defroster (because it's easier than using the ice scraper, right?)
   3. heater blower motor (I don't have a/c, so this isn't balanced out during warm conditions); heated seats/mirrors
   4. windshield washer pump (because it's easier than using the ice scraper, right? And for frequently cleaning off dirty road spray.)
10. More aerodynamic drag
    No, I'm not referring to the layer of snow you're too lazy to brush off the top of the car (though that would hurt mpg too).
    
    A vehicle’s aerodynamic drag is proportional to air density, and the density increases as temperature drops. For every 10 degree F drop in temperature, aerodynamic drag increases by 2% (source).