Even airlines get it: slowing down saves fuel!

I just saw this article on CNN the other day: Planes slow down to save fuel. Apparently airlines are finally starting to feel the pinch of higher gas prices, and are learning that going at a slightly slower speed can save significant money.

This isn’t a new observation: young folks today probably aren’t aware of the fact that the former national 55 mph speed limit was originally implemented in 1974 not for safety, but to conserve fuel in the wake of the 1973 oil crisis.

The reality is that air resistance becomes a major limiting factor on fuel efficiency at high speeds.  Roughly speaking, the drag force increases as the square of the velocity at high speeds, which means that driving 100 mph results in 4 times the drag force that driving at 50 mph causes.

I drive a hybrid Civic, and the dynamic mpg gauge on the vehicle gives me significant insight into how air resistance kills fuel efficiency.  I typically find that, driving steady at 55 mph, I can get 40-50 mpg.  Once I push the car up past 70 mph, and into 80 mph, I’m lucky to get 35 mpg.  At low speeds, the car can ‘coast’ for appreciable distances without using fuel, but at high speeds, the engine must constantly work against the drag force.

These numbers, over long periods of time, add up: driving a little slower on a day-to-day basis can save you some significant money in the long run.

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12 Responses to Even airlines get it: slowing down saves fuel!

  1. babs67 says:

    But I have the need for speed!

  2. “But I have the need for speed!”

    Maybe you should take up skydiving… 🙂

  3. I drive a Prius and have made similar findings, although I can drive as fast as 65 MPH and still get around 50 MPH if it’s not a long uphill stretch or anything. But you’re right: once you push past 70 MPH really starts guzzling the gas. And with gas topping $4 per gallon, that can translate, over time, into significant savings.

    Sometimes, though, you don’t have the luxury of slowing down. Just realize it’s gonna cost you! 🙂

  4. That should read “still get around 50 MPG….” as in miles per gallon.

  5. “I drive a Prius and have made similar findings, although I can drive as fast as 65 MPH and still get around 50 MPH if it’s not a long uphill stretch or anything.”

    That’s so like you Prius drivers to lord your superior mileage over us poor hybrid Civic drivers! 🙂

    “And with gas topping $4 per gallon, that can translate, over time, into significant savings.”

    When I got my hybrid (4 years ago), the U.S. was still at the point where buying a hybrid was purely an environmental choice: one couldn’t expect to make up the additional cost of the car in fuel savings for quite some time. I suspect that has changed, though and a hybrid is a pretty good investment, now.

  6. Janet Szabo says:

    I drive a Dodge one-ton MegaCab out here in Montana and driving at 55 mph on the highway I get 16 mpg. Driving at 70 mph (the speed limit here in the land of “keep the government off my back”) I get upwards of 18 mpg (which is pretty darn good for a 6.7L 350 hp engine). Why is it that I get much better gas mileage at higher speeds?

    Babs 67’s sister

  7. Janet Szabo wrote Why is it that I get much better gas mileage at higher speeds?

    Are you more aerodynamic at those speeds? 😉

    Maybe this a problem for Physics Guy to solve?!?

  8. Janet wrote: “Why is it that I get much better gas mileage at higher speeds?”

    Hmm… I don’t have a particularly good answer, probably because it’s more of an engineering question. My rough take: engine efficiency depends on speed in a complicated way, and each has a different ‘sweet spot’ based on its characteristics. As Jennifer above pointed out, she can do quite well at speeds in her Prius that would drain my Civic dry. It sounds like your Dodge has some sort of peak efficiency higher than most vehicles (this being relative, of course, because it has a much lower mileage than most sedans at any speed). In the end, though, if you go fast enough, wind resistance will start to have a serious impact.

  9. Janet Szabo says:

    Okay, I see what you’re saying. You probably should talk to my husband about this, because he knows all the specs of that Cummins engine and I’ve been told at what rpm’s I need to shift to be at the top of the power band and maximize efficiency blah blah blah. That truck is designed to pull heavy loads so it makes sense that the engine has been designed differently than a car engine.

    It would make me unhappy if I had to drive 55 mph because I know my mileage would suffer. Besides, being the fiancee’s sister, I also have the need for speed (but I don’t like flying, so you won’t find me flinging myself out of a plane).

  10. babs67 says:

    Janet Szabo wrote: “Besides, being the fiancee’s sister, I also have the need for speed (but I don’t like flying, so you won’t find me flinging myself out of a plane).”

    Just this weekend after seeing all the cool, fast cars in Iron Man, I told Dr. Skullstars I would love to be a stunt driver. Where on earth did we get that trait? Surely not our father, Captain Cautious…

  11. Janet wrote: “It would make me unhappy if I had to drive 55 mph because I know my mileage would suffer. ”

    I should add that I’m not advocating a national 55 mph speed limit, which would annoy me as much as the next person! This is more of a cautionary tale: if the U.S. doesn’t seriously start taking steps to conserve energy now, we’ll end up having to take more extreme and unhappy measures in the future.

  12. Janet Szabo says:

    You’re absolutely right. We should have started doing this 20 or 30 years ago. We’re trying to do our part out here by using biodiesel (although that comes with its own issues). As an added bonus, I get better mileage using it.

    I don’t think Captain Cautious liked to drive fast, but I suspect he would like my truck. 🙂

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