Follow up to Turning Your Car Off

A couple of weeks ago, I wrote an article about an easy way for you to increase your car’s gas mileage without having to alter any of your driving habits. To jog your memory, the tip was to simply turn your car off if you were going to sit at a stop light for more than ten seconds.

While I thought the tip made sense, it seemed to cause a bit of a backlash because it went against the common misperception that it takes more gas to start your car than it does to let it sit in idle for extended amounts of time.

Based on many of the comments I received due to this tip – in which some people stated that even idling for up to 15 minutes uses less gas than starting a car – I thought it would be a good idea to share how much I have increased my gas mileage by simply turning my car off at stop lights when I know I’m going to sit there for an extended period of time.

Before I start my “analysis” I’d just like to point out that my comparison will be based on the month prior to and the month following when I started turning off my car at long stop lights. While I know that this isn’t a complete apples to apples comparison due to all of the different variables, it’s as close as I’m going to come to putting together a “true” test.

In the month leading up to my decision to turn off my car at long stop lights, my car averaged about 31.25 miles per gallon, based on 1,406.4 miles traveled and 45.05 gallons of gasoline consumed. In the month following my decision, my car averaged about 33.05 mpg, based on 1,559.9 miles traveled and 47.2 gallons of gasoline consumed.

Because I didn’t take any long trips during either of the two months and almost all of the miles were piled up commuting to work, driving to the golf course or running various odds and ends, I can say without reservation that the main reason behind my 5.75% jump in fuel economy.

Granted, on a hot summer’s day, turning your car off at a stop light may not be the best thing in the world to do, but I would imagine that even if you were to do this just during your morning commute and later in the evening, you would probably add another 3% to your car’s gas mileage.

And as I’m sure you’re well aware by now, if each of us were to increase our fuel economy by 3%, we would save billions of gallons of gas (and billions of dollars) over the course of a year.

Comments

  1. I also do this, however, I wonder if a significant portion of your increased mileage in the second period comes from this. Let’s see if it’s plausible:

    If you had driven the 1559.9 miles in the second period with the miles per gallon (31.25) from the first, you would have used 49.9 gallons. Instead you used 47.2 gallons, a savings of 2.7 gallons. Now, I’ve had a Jeep Grand Cherokee Limited with a 4.7 liter engine and now have a Land Rover LR3 with a 4.4 liter engine. The Grand Cherokee used 0.38 gallons per hour at idle, the LR3 about 0.5 gallons per hour. Your car probably uses less than either of these at idle but let’s assume 0.4 gallons per hour. It would have taken you 6.75 hours of idling at stoplights to burn 2.7 gallons. In a 30 day month, that would be about 13.5 minutes of sitting at stoplights each day, every day. Assuming that a stop is an average of 40 seconds (don’t have data, just a guess), that would mean that, every single day, you were stopped at a little over 20 stoplights.

    Now, if you have a much smaller engine (I forgot what you drive), that would mean you’d have to be spending even more time at stoplights to have turning the vehicle off at stoplights be primarily responsible for your savings. Do you think that this is the case?

    Also note that these calculations assume absolutely NO extra fuel use on startup. I’ve searched the web and can find no figures for extra fuel used on startup, but the assumptions above are clearly the most favorable for fuel savings by engine shutoff.

  2. kevin craig says:

    Are you considering wear and tear on your car, especially the starter?

  3. Rob,

    Very insightful! I think you are correct in your assumption that smaller engines use less fuel while sitting in idle when compared to their larger counterparts.

  4. Kevin – Yes, I realize that there will be extra wear and tear on your car’s starter, alternator and battery. But, if you go back to the original article, these effects probably aren’t as bad as most people are lead to believe.

  5. In Japan, many people shut off their cars at stoplights. There are even people who will carry signs and parade them around at stoplights encouraging people to shut their cars off.

    There is the potential for extra wear on your starter, alternator and battery. If you’re concerned about that, talk to a reputable mechanic who’s familiar with your make and model of car. Not all starters and alternators are created equal.

    But at the very least, this should encourage you to park your car and go inside rather than sitting with your car idling at the drive-through at fast-food restaurants and banks.

  6. your car’s engine wares the most when you start it, even though you might save a bit in fuel, the long term damage you are causing to your engine, starter and the cars components in general greatly out weights the savings in fuel.

  7. I think any test or study that shows how most wear occurs during startup, is looking at cold starts. If you stop at a light after driving a warmed-up engine, you still have oil lubricating the engine when you start back up, reducing the wear compared to cold starts.

    So the next debate is which is better, quickly drive up to a stop light and shut off the engine, or coast up at a slower speed without needing to break to better time light changes?

  8. What qualifies as “an extended period of time?” In other words, what is the general threshold where shutting down becomes beneficial.

  9. Joe "Ted Village" Mesh says:

    Fist of all… two years later

    I’m in Japan. I was riding a bus last night. The driver turned the bus off while he let passengers off. He cranked it, and we drove off. It must have been off for seven point five seconds. I’m just not sure this works.

  10. At the risk of not closely reading Bob’s response:

    I calculated the amount of gas used per minute at 900 rpm (idle) to be .015 gallons, which comes to about a 4 and half cents at $3/gal, or less than buck a month if 20 minutes of idle time is saved. Calculation: With 34 mpg at 52 mph and 2000 rpm, one goes 34 miles in 39 minutes. So burn 1 gal / 39 min * 2000 rpm per revolution. Multiply that number by 900 (or actual idle speed). Hardley worth it.

    I suspect the increase posted was due to better driving habits, slower acceleration and maximum spead (unconcious or not) caused by the test itself, or enhanced gas conciousness.

  11. P.S. if you use the .015 gal/min result, and multiply it by 60 minutes, you find that the car burns a little than a 1 gallon per hour at idle.

    *This seems like pefectly reasonable number*. Other calculations that show much, much higher gas savings (article) would mean a car burns gas at idle at an incredible pace.

    The real test would be to actually measure gas use at idle, but a built gauge that shows gas level using gallons would be needed to make this practical.

  12. so is it better for people to shut their engines off during a 1 minute stop light or just leave it on idle?? does the feul consumption out weigh the car maintenance?

  13. another question can you still use your a/c while the engine is turned off? in this case just for a minute or 2?

  14. I’m just worried my car won’t start again. Not that it’s ever done that recently, but that didn’t happen to me once and few people are less forgiving that those stuck behind you.

    I do turn off the engine in drive throughs regularly. The reason I don’t get out of the car is I have a small child and it’s REALLY COLD OUT!

  15. To be honest not using A/C will save more fuel than nitpicky little things like this

    frankly I’ve always been of the opinion the whole ‘oh I can sit at idle for 15 minutes and use less fuel than I do when I start’ was a little ridiculous. If you assume 0.015 gal per minute to operate the motor at idle, 15 minutes later that’s .225 gallons. Do we really think that a car uses a FIFTH OF A GALLON TO START? that’s ridiculous. Absolutely ridiculous.
    maybe part of this myth comes from cold starts, which *do* consume a lot more gas. but your engine isn’t cold at a stoplight.

    • We need to work on the basic MPG of cars and not worry about ac as much. Try to tell America to get into a car in a traffic jam with their mother in law in August in Houston and they will run from fuel efficency like the plague. we need to make smaller cars with smaller engines and not worry about a car that will go from 0-50 in 5 seconds

  16. just to add on, that’s a little less than a quart of gas. Hint: most engines are about 1.5L which translates to .4 gallons. So the claim that you can idle for 15 minutes and be better off assumes that when you start your engine you *literally* fill it half-full with liquid gasoline. and not even that, because if I understand correctly how engine size works, at any given moment you only have .75L of *space* due to piston movement. Soo… we fill every single cc w/ liquid gas and start it. Yes?
    I’m sorry… it’s just so ridiculous.

  17. I started turning off my engine and now I am getting on average 65 miles to the gallon in a 95 Accord I cant beleive the difference! I can now afford to feed my cat

    • poopylollipop says:

      I didn’t feed my cat, fossilized him many years ago. Now i’m using his remains as fossil fuel in my coal powered truck, and i don’t have that stupid cat anymore!

  18. Gabriel says:

    I think that by turning off your engine at every stop light you may save some gas, but we should also consider the impact on the environment, since amount and quality of gases generated by turning on your engine may be worse that by those while idling.

  19. I have no idea which is better and am not sure if the data in Ricks blog is accurate or inaccurate. I’m interested in a reputable source of “any & all” savings by turning off the auto’s engine for 1.25 minutes. I also need the calculations or rationale to prove this. If you have the data/info. please email me at biojoe@comcast.net thanks!!

    • I think the most effective way to determine fuel usage during idiling would be to fill the car and then let it idle for a hour then refuel it to see how much fuel it has used. You could also do this in different conditions to see if climate or ac made much a substantial difference. I have rented a Prius and it seemed to really hate both heat and cold. have a feeling that its optional operating condition is probably in the 60′s for fuel economy. My Ford Focus hates hot weather with or without the ac and seems to keep getting happier the colder it gets.

  20. This is not a good idea. You may save a few pennies in gas but you will make up for it many times over by wearing your engine and starter at a much faster rate.

    Startup is the most brutal time for an engine so if you take a car from 5-6 starts/day to 80-100 starts/day you are going to wear the engine out.

    Keep it idling and turn off your A/C compressor at red lights if you want to save a little, but don’t start it over and over and over. You’re just asking for trouble.

    • I think the starters are designed to operate with a expectation of starting a set number of times by the manufacturer. Say once every ten driven miles. You probably would not ruin a starter if u just have the vehicle a few years and bought it new but if you drive towards the full lifetime of the vehicle which is probably about 200000 thousand miles I think you will find a higher failure rate than if you had been starting it normally.

      starting every ten miles it would be 10,000 starts per 100,000 miles driven
      starting every 5 miles would be 20,000 starts per 100,000
      starting every 2.5 miles would be 40,000 starts per 100,000

      work the numbers for driving 200,000 miles and you start with 20’000 starts, at the end of it would be 80,000 starts. not sure if anyone designs a starter to reliable work to those numbers.

  21. If you want to save fuel then don’t wait for a rain storm to check your lights and wipers. Make a habit of checking both at regular intervals, when the weather is good like when you fill up at the gas station. Seeing and being seen are critical anytime, but especially while driving in the rain. Functioning lights are important not only for your ability to see, they’ll help other drivers see your vehicle. Windshield wipers should be replaced at the first signs of trouble, like streaking or noisy operation, in order to provide the best visibility while on the road.

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  23. I’ve had trouble finding definitive information on subjects as this, but I can tell you from first-hand experience that turning off your engine WILL save you fuel.

    I do it on all of my commutes and regularly get 3+ mpg more on average. The myth probably arose when cars were carbureted and/or regarding cars starting up when it’s really cold outside. This site seems to be the most trustworthy that I can find, and even addresses myths about additional wear and tear during startup:

    http://www.consumerenergycenter.org/myths/idling.html

  24. sherman says:

    Ok but what u also fail to relize is what happens to all the gas in the motor when u shut it off? Ya it gets wasted and dumped in the exhust when the motor returns and u cause a lot of dame to the starter and alt and battery which for almost every vehcicle is expensive. So the fuel increase u get from turnin it of is wasted from the repair bill for the damage the car causes and it costs u more really to turn the car off and on

Trackbacks

  1. Anonymous says:

    Does it Take More Gas to Start Your Car or Sit in Idle?

    One of the common beliefs among drivers is that it takes more gasoline to restart your car than it does to sit in idle for a couple of minutes. Is this belief costing you hundreds of dollars a year?

  2. [...] Turning off your car engine instead of idling really does save you gas. See also: Idling Gets You Nowhere. [...]

  3. [...] guy tested it over a month and saved gas by turning off the engine. The Environmental Defense Fund endorses the [...]

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