To Idle Or Not To Idle? What's Best For Car, Gas Mileage?

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There are two kinds of traffic, neither of which is particularly pleasant.

The first is moving traffic. It's slow, tedious, and something many of us are unable to avoid on our daily commutes. Movement is movement though, which makes stationary traffic a hundred times worse.

Not only are you not getting any closer to your destination, but as you sit there you may as well be chucking dollar bills into the breeze, as your engine drinks through its fuel, wasting money, making noise and causing pollution.

The question is then, should you switch it off? TheGreenCarWebsite reports on a scheme in London getting drivers to do just that.

Environmentally, the default answer should be yes. Not only does switching off your engine at a standstill use less fuel - and therefore, cost you less in gas - but your car automatically becomes zero-emissions.


There have always been one or two factors discouraging people.

The first is a concern that leaving your engine running actually uses less fuel than it would by stopping and re-starting each time. This might have been true in the days of carburetted cars, but modern engines are much less fuel-heavy on startup, and even leaving the engine running for ten seconds in traffic would use more fuel than stopping and re-starting.

Engine and starter wear, and battery draining are further concerns. Once again, some older vehicles may benefit from being left running, but studies by Transport for London have found that modern cars are barely affected, if at all - battery drain is minimal even after hundreds of re-starts. Engine wear is insignificant, and potentially lower than leaving it running for longer periods of time.

There are concerns about the engine not reaching full temperature or the heater not working when the car is off. Many factory-fit stop-start systems won't operate until the engine is at running temperaure, so there's little stopping you from doing the same.

As far as heating is concerned, a warmed engine will allow the heater to work absolutely fine for a fair while - simply turn the engine off, but keep the ignition on and the fan blowing.


Reduced fuel use is the main benefit. Several manufacturers have complained that the EPA's testing system doesn't accurately reflect the benefits of a stop-start system, which many are now fitting as standard, to city fuel economy.

To illustrate the difference in a system that does, the European cycle, we can compare the fuel economy of Smart's ForTwo.

2011 Smart Fortwo

2011 Smart Fortwo

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In the U.S, Daimler offshoot Smart sells the 1-liter, 70-horsepower ForTwo. This gets MPG ratings of 33 mpg city, 41 highway and 36 combined.

It also has a European equivalent. At launch, this managed 38.5 mpg urban, 58.8 extra-urban and 50 combined - cycles that are roughly equivalent to the EPA's city, highway and combined ratings - but represent the usual 20-30 percent difference between EPA and European ratings.

In 2009, the range was updated, and the 70-horsepower ForTwo gained Smart's "Micro Hybrid Drive" system - essentially, a stop-start system. While the 58.8 "extra-urban" figure remained identical, the urban figure shot up to 45 mpg - an improvement of 6.5 mpg or around 17 percent, simply from the amount of time during testing the car spends off, rather than idling.

A similar improvement to the Smart's EPA figures would raise city MPG from 33 to 38.5 mpg - a worthwhile reduction in gasoline usage.

While there are flaws in the Euro system (the top speed in the highway equivalent is relatively low, hence the high MPG figures), it certainly illustrates the benefits of turning the engine off when at a standstill.

Of course, all the time you're not wasting fuel going nowhere, you're also not contributing to air pollution. An engine not using any fuel whatsoever is the cleanest engine of all.

And though the proportional saving for larger vehicles may be no more than it would for something like a Smart, the overall saving for less efficient engines would be much greater, as they use more fuel in the first place.

Benefits for all

While London's scheme is aimed mainly at old, particulate-producing vans and black cabs to improve the English capital's poor air quality - particularly in wishing to avoid the pollution problems from China's Olympics in 2008, when the games hit London this year - there are benefits whatever vehicle you drive.

Of course, there are clean air benefits even when you aren't driving too - it's much more pleasant to walk around a city with lower noise and pollution levels.

So next time you're sitting idle in a traffic jam, think about whether your car is doing the same.

Stuck for a while? Turn it off, and enjoy the silence, the fuel savings, and the cleaner air.


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Comments (3)
  1. I have a ScanGuage II mounted in my Toyota Corolla and whenever I am sitting in traffic, I can see the average MPG for the trip slowly tick downward and it is depressing. Not sure switching off is practical as traffic tends to creep a little even at traffic lights and people can't stand to see a gap opening up in front of your vehicle.

    But the benefits of hybrids become very clear in these situations. My Prius can just creep along without turning the gasoline engine on, very nice.

  2. Yeah, that is a problem - in some types of traffic, it's inconvenient to turn the car off and back on again. More useful at stop lights that stay red for a while, or big jams.

    And yes, certainly an advantage of hybrids.

  3. I wonder how many people are aware that idling is illegal in many US states.

    In NJ & NY for example you're limited to 3 minutes of idling before a sharp fine can come your way… except, when you're in a stationary line of traffic.

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