An easy change to improve your city gas mileage by six percent

Last Updated on April 15, 2017 by Dave Farquhar

Would you like to improve your gas mileage in stop-and-go traffic by five or six percent without spending any money and without changing what you drive?

Shut off your car instead of letting it idle. I got this from Daily Fuel Economy Tip.This is old advice (I first heard it back in 1990 when I started driving), and it’s controversial advice, but in these times of high gas prices and a shaky economy, the advice is going around again. So I did some digging.

First, the counter argument against it. The more you start your car, the more wear and tear you’ll have on your battery, starter, and alternator. There’s some truth to that, but the Canadian Department of National Resources says the wear and tear by stopping and starting rather than idling amounts to $10 a year. Those are Canadian dollars, but the value of the Canadian and US dollar are so close, the estimate is valid in the States too.

Still unsure? Not all alternators are created equal. Some years ago, I had to buy a new alternator for my 1992 Dodge Spirit. It cost me about $90. The guy at the auto parts store told me that the alternators on some cars cost $25, but expensive alternators last longer. So one phone call or web search can give you a good indication whether this is a good idea. If an alternator for your car costs $25, you should be more wary than if an alternator for your car is expensive.

If you’re still not sure, bounce the idea off a reputable mechanic who’s familiar with the make and model of your car. My wife and I take our Hondas to an independent repair shop that’s been specializing in Hondas for more than 20 years. You bet I’ll be talking this idea over with him. I suspect it’s OK to do, because shutting your car off at stoplights is common practice in Japan–the Japanese government started encouraging the practice 10 years ago, in June 1997.

All that said, I had no trouble whatsoever finding advice saying to do this. Finding the counter-arguments is much harder. The only place I could find any were in the comments on web sites encouraging the practice, from people giving anecdotal stories about their cars not starting after doing it.

Assuming the stories are true, the problem is preventable. The Advance Auto Parts down the street from me will test your battery, starter, and alternator for you for free. Most other auto parts stores have similar services. If your battery is questionable, replace it with the biggest, burliest, most testosterone-laden battery they have that will fit your car. The price difference is likely to be less than $20. If the starter and/or alternator is questionable, then you have a decision to make, but keep in mind a questionable starter or alternator will eventually fail under normal driving conditions too.

I’d always heard that starting your car takes less gas than idling for 30 seconds, but at least one web site is setting that bar much lower, at 8 seconds. The state of Oregon sets the bar at 10 seconds (my apologies for using a secondary source–the primary source is no longer available). The Canadian government agrees with 10 seconds. Both are much lower than what I’d heard in the past.

For that matter, one “hypermiler” who gets an average of 59 miles per gallon out of his bog standard Honda Accord even shuts his engine off at certain times while moving–such as when he’s able to get behind a large semi truck. He warns in the comments for that article that some of the things he does can get you killed if you don’t know what you’re doing.

I think if he routinely shuts his engine off and restarts it while moving, I can probably safely do it at stoplights and settle for more modest gains in gas mileage.

So what do you stand to gain by doing this? I burn about 12 gallons a week. Given 52 weeks in a year, I would stand to save 31.2 gallons a year by using this practice, assuming a savings of five percent. Even if gas were to drop to slightly under $2 a gallon, that savings wouldn’t be chump change. It’s considerably more than the $10 the Canadian government says I’d spend per year on wear and tear. If you drive a bigger vehicle than I do, the savings are bigger.

And over the long term, your savings could be more than just five percent. The Canadian Department of Natural Resources says the excess wear on your spark plugs from idling can decrease your fuel economy by 4-5 percent.

Even if the idea of shutting your car off at stoplights doesn’t appeal to you, at the very least, this should encourage you to shut your car off at the ATM, and either shut your car off while waiting at the drive-through at restaurants and at the bank, or better yet, park your car and walk inside. (From my experience working in restaurants, they’re a lot more likely to get your order right if you go inside than at the drive-through anyway, because it’s a lot easier to understand you in person, because there’s less rushing around at the counter than at the window, and there are a lot fewer interruptions at the counter, both of which make it a lot easier to concentrate.)

And this little bit of research definitely confirms my suspicion. A couple of weeks ago I saw a Hummer H2 sitting idling in the parking lot at the mall. When I came out 20 minutes later, the Hummer was still there, its engine working hard doing nothing. I should have left a note on the windshield.

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One thought on “An easy change to improve your city gas mileage by six percent

  • April 21, 2011 at 11:47 pm

    Does R. Collins Farquhar IV agree with this article? I’d like to hear what he has to say. Does his driver kill the engine on his 1967 Rolls-Royce?

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