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You're not imagining it—cold does reduce mileage

If it seems like your fuel economy has been lower than normal this winter, it's not your imagination.

Like people, cars find it harder to get through a cold winter, which is especially true for the latest generation of battery-based vehicles.

New research from the Oak Ridge National Laboratory found that some models can lose as much as a third of their fuel economy when the mercury drops to 20 degrees. And the colder the weather or shorter the trip, the bigger the drop.

A surface street in Brooklyn, N.Y., on Feb. 13.
Getty Images
A surface street in Brooklyn, N.Y., on Feb. 13.

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"Cold weather and winter driving conditions can reduce your fuel economy significantly," Bo Saulsbury, in Oak Ridge's research group, said in the report on winter driving. "Fuel economy tests show that, in short-trip city driving, a conventional gasoline car's gas mileage is about 12 percent lower at 20 degrees Fahrenheit than it would be at 77 degrees Fahrenheit."

The research center took the rated fuel-economy results of 600 conventional vehicles and 14 hybrids under "normal" warm weather conditions and compared them to tests at 20 degrees.

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The worst drop came from battery-based vehicles, which saw their typical mileage drop by between 31 percent and 34 percent. That means a 45 mpg model such as a Ford Fusion Hybrid might deliver only 30 mpg when the temperature plunges.

But even a gas-powered car that normally gets 30 mpg might struggle to deliver 24 mpg during short trips, when mileage drops the most, according to the Oak Ridge researchers.

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Lower temperatures are just one of the reasons for reduced fuel economy. The lab pointed to other factors, including:

  • Icy or snow-covered roads, which can decrease traction. Fuel is wasted when the car is literally spinning its wheels.
  • Slowing down on slick roads, as vehicles are typically designed to deliver their best mileage at highway speeds.
  • Operating in four- or all-wheel-drive.
  • Thicker oil and other vehicle fluids, which increase internal friction and the time it takes a car to reach optimum operating temperature.
  • Using more energy to power blower fans, defrosters and seat heaters.
  • Winter gasoline blends, which tend to have lower levels of energy per gallon than summer blends.
  • Letting a vehicle idle—getting zero mpg—to warm it up.

Then there's the battery. Even in a conventional vehicle, it's likely to be less efficient when it gets cold, requiring the alternator to run more frequently. And in a hybrid, plug-in or electric vehicle, a cold battery will hold less energy, limiting both range and energy efficiency.

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According to Oak Ridge research, drivers can take a number of steps to improve their mileage during the winter. For example, park the car where it can stay warm and combine trips rather than make a lot of short ones, so the engine and fluids are fully warmed up.

Other mileage-friendly suggestions include:

  • Using the oil the manufacturer recommends for winter.
  • Limiting the amount of time spent warming the vehicle up, as it will actually heat up faster while driving.
  • Checking the tire pressure frequently. This falls as temperatures drop, tires aren't as energy efficient when they're low and may not get an adequate grip.

For a plug-based vehicle, such as a Chevrolet Volt or a Toyota RAV4 EV, preheat the cabin, Oak Ridge researchers said. That way the car warms up while it's plugged in rather than while it's being driven.

By CNBC Contributor Paul A. Eisenstein. Follow him on Twitter @DetroitBureau or at thedetroitbureau.com.

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