Save Money on Gas - Drive More Slowly

by: James Hamilton posted on: June 22, 2008 | about stocks: DBO / OIL / USO    

Environmental Economics and The Energy Collective are among the many voices recently advising consumers they could save gasoline by driving more slowly. I was curious to take a look at the evidence behind such claims.

Source: U.S. Dept. of Energy.
speed_vs_mpg.gif

Air resistance increases the faster you travel, which might lead you to think that higher speeds always require more fuel. However, your car's engine is designed for maximal efficiency in converting fuel into motion when you drive at higher speeds. As a result, the typical car gets much better gas mileage if you drive it at 45 mph instead of 15. However, at speeds above 60 mph, the wind resistance becomes a dominant factor, and miles per gallon for most cars starts to decline significantly if you drive faster than 60.

The basis for the graph above and many of the quantitative claims one finds in newspapers and blogs appears to come from a 1998 study, whose raw data can be found in Table 7.23 of the DOE's Transportation Energy Data Book. This reports miles per gallon as a function of speed for 9 automobiles on the road in 1997.

In principle, this is a straightforward thing to measure-- you drive the car around a fixed track trying to keep it at, say 65 mph, and measure the fuel consumption. Then you do it again at 75 mph and calculate the change.

Consumer Reports and Edmunds.com have run analogous trials with similar results to those reported by DOE. The first column of the table below summarizes those results in terms of how many more gallons of gas it would require to drive 100 miles at a speed of 75 rather than 65 in each specified vehicle, while the second column expresses these same numbers as the percent by which you'd reduce your fuel consumption in that type of car if you reduced your average speed from 75 to 65.

For example, with the 20% fuel savings you'd get from slowing down in the Olds 88, it's as if you could buy your gasoline for $3.20 a gallon rather than $4.00 and still do just as much driving.

Consumer savings from driving at average speed of 65 rather than 75

vehicle
gallons per
100 miles
% reduction
in fuel cost		equivalent hourly wage
nationalSan DiegoOklahoma
1988 Corsica0.7417.42%$14.66$16.45$13.47
1993 Legacy0.4813.77%$9.53$10.70$8.77
1994 Olds 880.8320.00%$16.62$18.65$15.28
1994 Cutlass0.6313.60%$12.55$14.09$11.54
1994 Chevy pickup0.9416.97%$18.70$20.98$17.19
1994 Cherokee0.5410.33%$10.78$12.10$9.91
1994 Villager0.5111.46%$10.20$11.45$9.38
1995 Prizm0.6017.01%$11.98$13.45$11.02
1997 Celica0.4215.40%$8.35$9.37$7.67

Of course, there's also an economic cost of driving more slowly, which is you'll take longer to get where you're going. Again considering this idealized experiment of driving a distance of 100 miles at exactly 65 mph rather than 75 mph, it would take you

100 miles x (1 hour/ 75 miles) x (60 minutes/hour) = 80 minutes

to get there at 75 mph, but 92 minutes if you only drive at 65. So you'd spend an extra 12 minutes to save 0.94 gallons if you drive a 1994 Chevy pickup for 100 miles.

How much money that saves you depends on how much you pay for gas. My table reports three reference values, first a "national average" based on the current average U.S. price of $4.09/gallon, the second a high-priced community (my home San Diego, where it's now $4.59), and the third for one of the cheapest spots in the country (Oklahoma City's $3.76/gallon)-- those prices come from NewJerseyGasPrices.com.

You can then convert that to an hourly wage you could consider yourself to be earning for driving more slowly. For example, if you drove that Chevy pickup for approximately 500 miles, you'd do an extra hour's worth of driving, but save yourself $18.70 if you were paying the current national average retail gasoline price.

And, by the way, that $18.70 per hour is entirely tax-free income from your point of view.

Having done these calculations, I should point that you can't always drive on San Diego's freeways at 75 mph. If you try to maintain that speed, you will be braking and accelerating more than someone who tries to keep it at 65. Each time you put your foot on the brake pedal, that's costing you money as well, in that you're killing the momentum provided by the fuel you've already paid for and burned. DOE separately claims that motorists could reduce their fuel bill by 5-33%, depending on their existing driving conditions and habits, by trying to use the brake pedal less and accelerate more slowly.

Driving more slowly should also reduce your carbon footprint and any other pollutants by the same amount indicated in the second column above, and should lessen the likelihood and severity of collisions. And most urban highways prohibit speeds over 65, so you're exposing yourself to risk of fines and higher insurance rates if you drive at higher speeds.

Even without these added considerations, however, I would think that many people, if they knew that the immediate financial rewards were on the order of the numbers given above, might choose to drive more slowly. In which case, it is perhaps a public service to help call such numbers to people's attention.

Which, in case you were wondering, is why I wrote this post.

This article has 11 comments! Add yours below...

James Hamilton

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This article has 11 comments:

  •  
    Paul A.
    Jun 22 06:12 AM
    Why has no one, not George Bush, Barack Obama, John McCain, or anyone else who fancies himself/herself a leader gotten behind lower speed? It is the single thing that US consumers can do to reduce gasoline consumption, and theoretically, gasoline prices in the near term. Seems like a no-brainer to me.
  •  
    User 151885
    Jun 22 08:17 AM
    No one has to force or tell drivers to slow down. It's a naturally occurring process that has already begun!
    Market's do not need interference. They work on their own through evolution and the striving for survival and then improvement. A solution or solutions will evolve that have not yet even been conceived.
  •  
    fatcat
    Jun 22 09:52 AM
    Good article..I knew there was a savings,but not how much..I wonder how this relates to more modern cars?
  •  
    whiplash1
    Jun 22 10:16 AM
    They wont tell you this because the highways will get more congested. Imagine everyone driving at 30mpg; people will be on the highways longer period of time. There are about 3 million more drivers in this country every year. Thanks to immigration and high birthrates. Stop breeding and there will be plenty for everyone. 1 child per family MAX or there will be dire consequences
  •  
    Jackal
    Jun 22 10:58 AM
    How many kids do you have, whiplashi...?
  •  
    Shaggieman
    Jun 22 11:35 AM
    Nixon implemented 55 mph max speed back when and it seemed to work well. Folks were upset at first but got used and it did save. Most travelled at 65 mph before they got ticketed.
  •  
    datalink1995
    Jun 22 02:29 PM
    My Website
    I have a 1998 Dodge Grand Caravan. Before the sharp rise in gas I was a “low end” speeder, exceeding the posted limits by about 5mph and getting 22.56 mpg on a combined city and interstate highway travel. I then slowed down to going 5 mph UNDER the posted limits on the interstate (staying in the right hand lane at all times) and maintaining the speed limit in town. My mpg went up to 24.7 mpg! That is 9.49% increase. I’m sure that on a long trip using interstate only I’ll get over 25 mpg.

    I also did all of the other things in the article, like anticipating stop lights, a light foot on acceleration (if you have a tachometer, use it to get out of the low gears). What the author said about the most economical speed range was right on. 45 to 60 is the most economical.

    Also make fewer trips by planning and grouping your trips around town.

    And driving around your own city the time differences are insignificant.
  •  
    paulk8756
    Jun 22 04:30 PM
    I remember "Drive 55" well. It was right up there with Nixon's great ideas like his proposed "national minmum incomes" policy. That one was DOA even in the then liberal Democrat Congress of the day.

    But getting back to Drive 55, I have no doubt that's high up on the Democrat agenda (...along with a new national income policy) if they can get Obama elected.

    Thank God that's looking like less and less of a certainty.
  •  
    TinyTim
    Jun 22 05:01 PM
    Nixon's Double Nickel ranks right up there with yanking the gold standard. Only recently did cars stop having the 55 speedometer digits in red. Get crazy & go to: hypermpgs.com or cleanmpg.com

    The opportunity cost of driving slowly is fascinating when applied to involuntary situations like rush hour stop-and-go congestion multiplied by thousands of drivers. It makes the price of creative solutons or even more permanent lanes look attractive.
  •  
    ART005
    Jun 22 05:38 PM
    whiplasdh1,
    There's a very clear chart at the top that shows mpg efficiency at 60 mph equal to 30 mph so 30 mph highways never suggested. In fact some (generally smaller) cars are geared such that 60 mph is more efficient than 55 mph. So up to 65 is very reasonably efficient and as the atricle says by 75 mph there are costs that people might prefer not to pay.
  •  
    whiplash1
    Jun 26 01:43 PM
    The Jackal...

    I have NONE..So other skids can have more.


    On Jun 22 10:58 AM The Jackal wrote:

    > How many kids do you have, whiplashi...?
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