Reduce speed a little. Save a lot.
by Terry Laughlin

Posted on March 10th, 2010

The car I’m driving now – a VW Jetta TDI wagon – has great fuel economy. A careful driver can get up to 50mpg. One of its great features is an option to display moment-by-moment fuel consumption. When accelerating from a stop it may be as low as 8mpg. When I lift my foot from the gas pedal and coast on momentum it can go as high as 200mpg.  That provides a powerful incentive to drive more economically. But it has also provided an insight into how reducing speed by relatively small amounts can yield surprisingly large fuel savings. And the feature on my GPS which estimates arrival time at a destination I’ve programmed in shows how little time it costs me when I drive with a lighter foot and a much less thirsty engine.

Twice a week I drive from Coronado, where I’ve been staying, to LaJolla Cove, where I swim. The trip is about 18 miles with 14 of that on I-5. The first few trips I drove  the interstate part at 70-75 mph. I noticed that my fuel consumption for those trips was 34-36mpg. When I reduced my speed to 60 mph, fuel efficiency improved to 44-45mpg. I reduced speed by about 17% and improved fuel efficiency by 25%. And my trip duration increased by just two minutes — from about 22 minutes to 24 minutes.

The reason why efficiency increases by 50% more than speed decreases is the geometric effect of wind resistance. A small reduction in speed can mean a significant reduction in energy cost.

There’s a corollary here for swimmers — particularly distance swimmers, and this goes double for triathletes. Because the primary cost of swimming is in overcoming water resistance, and resistance goes up far more than speed, it takes a LOT more energy to swim a little faster.

This means that you can save a LOT of energy by moderating your pace a little bit.  And the energy you save can help you hold that pace much longer. Or, in a triathlon, it could help you gain a lot more time on the run (or bike) than you give up on the swim, since – unlike swimming – it takes only a LITTLE energy to run a little faster.

And by practicing Total Immersion Swimming, you’ll probably recover the speed you sacrificed in short order.

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6 Responses to “Reduce speed a little. Save a lot.”

  1. Alan says:

    Great ‘analogy’. I’m always looking for a way to drive this point home. So many feel that “conditioning”, to absorb that extra effort is enough!

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  2. Christian says:

    I understand your message. But what if you are facing some fairly significant currents — such as crossing the Chesapeake Bay or Alcatraz — then you have to keep up a pace in order to stay within the course. Lowering the pace may not be an option.

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  3. There certainly may be occasions when an increase in pace is called for. The question is how long will that need to be sustained and can it be sustained. The advice on slightly lowering speed to find a more sustainable pace is directed to the great majority of open water racers who swim at an unsustainable pace early then spend the bulk of the race swimming more slowly. Many people would find their overall time better if they started out a bit more slowly.

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  4. haschu33 says:

    Great analogy! Yes, air resistance and water resistance both increase by the power of 2 if you double speed, if I am not totally wrong here.
    I am driving a VW T5, that is the latest version of the VW van, not available in the US, as far as I know. Also with a Diesel TDI engine. My mpg (had to convert my liters/100 km to miles/gallon) can come up to 30, usually is about 25. Regarding that it is the camper version with a weight of more than 2,5 tons and that can still run at 180 km/h (appr. 110 mph) this is quite a good value. But a mpg of 50 is not accessible for me. I don#T know about the US, but the mpg (or liters per 100 km) is one of the most important issues in cars here, more than the speed it can run at.
    I am delighted to hear that there are people in the US who are concerned with energy saving not only in swimming but also in gasoline!
    I did some tests when I had my Volvo V70 on my usual drive to work and back. It is drive of about 45 km (=28 miles) with about 17 to 18 miles of Autobahn, most of it without speed limit. Once I drove not faster than 100 km/H (a little more than 60 mph), the other time whatever was possible meaning up to 120 mph and more. The time saved was 3 minutes in an appr. 45 minutes drive. The usage in gas (and my nerves) was considerably higher, about 20-30 % more. Basically not worth it.

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  5. Haschu – Certainly a useful insight for distance swimming. I’ll be mindful of that on April 17 when I swim 24 miles in the Tampa Bay Marathon.

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