AP Photo/Hazleton Standard-Speaker Ellen F. O'Connell

Note: See also my follow-up to this article, "This is why people still think they should idle their cars in winter."

We've all heard the idea: In winter, your car needs a little time to warm up before you can drive it. And that's why across the United States, people who live in cold and snowy places -- and especially those whose cars have remote starters -- often fire up their engines long before they start driving. Heck, they might even start the car from the kitchen in the morning, and only then start the coffee brewing.

But it turns out that this idea of idling your car during the winter is just wrong. And so are the many, many Americans who believe it -- one 2009 study found that on average, Americans thought they should idle for over 5 minutes before driving when temperatures were below 32 degrees! 

Like many misconceptions, the idea behind winter car idling begins with a kernel of truth. Cars do get worse fuel economy when it's really cold out -- they are at least 12 percent less fuel efficient, according to Environmental Protection Agency and Energy Department. And it does take longer for the engine to warm up and reach an optimal driving temperature in cold weather.

Moreover, older cars -- which relied on carburetors as a crucial engine component  -- did need to warm up to work well, according to several auto industry experts. Without warming up, the carburetor would not necessarily be able to get the right mix of air and fuel in the engine -- and the car might stall out. During the 1980s and into the early 1990s, however, the auto industry did away with carburetors in favor of electronic fuel injection, which uses sensors to supply fuel to the engine and get the right air and fuel mix. This makes the problem of warming up the car before driving irrelevant, because the sensors monitor and adjust to temperature conditions.

Idling in winter thus has no benefit to your (presumably modern) car. Auto experts today say that you should warm up the car no more than 30 seconds before you start driving in winter. "The engine will warm up faster being driven," the EPA and DOE explain. Indeed, it is better to turn your engine off and start it again than to leave it idling. (As many readers pointed out after this post was first published, it's always important to be careful driving in winter, and clear your windshield of any ice.)

So idling does nothing for your vehicle, but it does have several big (and avoidable) costs: Wasting fuel, and giving off greenhouse gas emissions and other types of pollution.

To show as much, Natural Resources Canada -- the energy and resources agency of a cold country that also has serious idling problems -- ran an idling experiment, freezing three cars to minus 18 degrees Celsius and then driving each one the same distance. Sometimes the cars were idled five minutes before driving, and sometimes 10 minutes. The result was that the more idling time, the more wasted fuel.

"The test results showed that with a 5-minute warm-up total fuel consumption increased by 7 to 14 percent and with a 10-minute warm-up total fuel consumption increased by 12 to 19 percent,"  the agency reported.

The Department of Energy's Argonne National Laboratory, which has also conducted much research on idling, reported that "idling fuel consumption is, of course, linear with time, and increases with engine size":

Source: Argonne National Laboratory.
Source: Argonne National Laboratory.

Or to put it more bluntly: Whereas newer cars are constantly improving the miles they get per gallon driven, idling will always be stuck in place -- using up gas, but getting no miles for it.

But it's not just fuel waste, it's the accompanying emissions. What does it look like when you have a whole population of people -- or at least the northern belt of a country like the U.S. -- idling their cars in winter?

A 2009 study in Energy Policy tried to calculate the consequences. The researchers found that, overall, all types of vehicle idling -- idling in winter, idling while waiting for someone or something, and idling in traffic -- contribute a staggering 1.6 percent of all U.S. greenhouse gas emissions.

That number is "almost double the total emissions for the iron and steel manufacturing industry," the paper noted. (In fairness, since the study was published vehicle fuel economy has improved, and new vehicle greenhouse gas emissions have declined, thanks to new regulations. So especially for new vehicles, this may somewhat blunt the overall effects of idling.)

That is not to say that all idling should be stopped immediately. Some idling -- particularly in traffic -- may be unavoidable. But the other two categories of idling -- in winter and while waiting -- make a lot less sense. And the study found that they account for nearly half of all U.S. greenhouse gas emissions attributable to idling.

And no wonder: When 1,300 Americans were surveyed about idling for the study, nearly half reported both idling their cars for longer than 30 seconds to warm them up and idling for more than 30 seconds because of waiting. Indeed, the average amount of time that respondents thought you should idle your car before driving, when it is lower that 32 degrees Fahrenheit outside, was 5.01 minutes! And since that's the average, many people thought you should idle for a lot longer than that.

"These values indicate that beliefs about how much idling is appropriate or desirable are highly distorted," the authors wrote.

The study found that if people would just knock off unnecessary idling of this sort, then consumers as a whole would save $5.9 billion per year on fuel costs (based on the cost of fuel in 2008). The saved emissions, the study noted, would be "larger than the emissions from the soda ash, aluminum and limestone industries combined."

Idling behavior, the paper concluded, is "worthy of policymakers' attention." Some have taken note. For instance, often-freezing Minneapolis has an anti-idling statute that restricts all non-traffic idling to three minutes per hour (with some exceptions). Anti-idling laws across the country vary, but some localities follow a similar course. So idling isn't just pointless -- beyond a point, it may even trigger a hefty fine.

Meanwhile, technological advances, and the push for ever greater fuel economy, are even starting to help deal with the most unavoidable type of idling: Idling because you're stuck in traffic. Vehicle start-stop technology literally shuts down the engine when your car is stopped, and automatically switches it on again when you start to drive again. This technology tends to be found in hybrids but has spread to other cars as well. GM now boasts that 97 percent of buyers of a 2014 or 2015 Chevy Malibu bought a car with start-stop technology.

So, it's hard to see any redeeming value to idling your car in winter. For the final word on the dumbness of this practice, let's turn to the late Tom Magliozzi, the unforgettable co-host of NPR's "Car Talk." As he put it to a Boston listener named Lisa, who had asked about her boyfriend's conviction that you need to idle up to 10 minutes in winter:

"Dear Lisa's Boyfriend: You have your head so far up your tailpipe on this one, it may be coming out your air intake."

Update: Many readers have responded to this piece by raising some good and interesting points. In particular, some folks have noted that they idle their cars in winter not for the sake of the engine, but rather for comfort (warmth) or because it helps in defrosting. While this article was focused solely on the energy and fuel consequences of idling, I acknowledge these other reasons. That said, the research cited above does suggest that many people think idling is necessary for their car's engine, not for comfort or safety. So please, click the links if you have more questions about idling (here's EPA and DOE, and here's Argonne National Laboratory), and have a happy and safe driving experience this winter!

Update 2: I've published a follow-up to this article, entitled "This is why people still think they should idle their cars in winter."