Slow Down and Save Fuel

woman fuel filler gas premium gas

On the radio show, I hear from people all the time who are disappointed in their fuel economy on their new cars.

Outside of a couple of isolated cases recently with Hyundai and Ford, I find the mileage estimates on vehicles today to be extremely accurate, but there are some variables.

We know that the automakers themselves rate the vehicles, and the government spot checks them for accuracy. Since fuel economy is such a huge factor in decision making on new vehicles, we know the tests are done in near-perfect conditions.

My experience with people who complain about fuel economy is that often, they haven’t let their vehicle break-in yet. With some of the newer vehicles, that doesn’t occur until 10,000 miles or so. Also, many trust the fuel economy reading on their dash, which can often be off. If you want a true reading, you should check it the old fashioned way. The biggest thing I see, however, is poor driving habits, and in particular speed.

So, how much does your fuel economy fall — and how much does your fuel cost rise? — if you drive at 60 m.p.h. rather than 50? How about 70 m.p.h.? 80?

Check this out…there is a 41% decrease in fuel economy from 50 m.p.h. to 80! That’s like paying $1.38 more per gallon of gasoline, according to the U.S. Department of Energy’s website

Until recently, nobody had quantified the effect real-world speeds have on fuel economy, despite the fact that most people hit those speeds on the highway every day. The differences from one vehicle to another are surprising, according to Brian West, a development engineer who worked on the study at DOE’s Oak Ridge National Laboratory in Tennessee last year.

The smallest drop in fuel efficiency was 6.9% from 50 to 60 m.p.h., while one vehicle lost a surprising 26% between 70 and 80 m.p.h. Other vehicles saw their fuel economy decrease as much as 18.3% at 60 and as little as 10.8% at 80. There wasn’t much correlation between decreasing fuel economy and the vehicles’ frontal area and aerodynamic drag. That’s one of the surprises the DOE team is still trying to figure out, because it seems to violate a law of physics. This has to be distressing to the automakers’ engineers too, they work hard to make cars more fuel-efficient all the time.

The study compares 74 vehicles’ fuel economy at 50, 60, 70 and 80 m.p.h. All the vehicles were tested on chassis dynamometers using Society of Automotive Engineers standards. DOE tested 24 of the vehicles in Oak Ridge while Chrysler provided data from 50 vehicles it tested at its labs in Michigan. Vehicles of all body styles, drivetrains, and major manufacturers were represented.

While the figures will vary from one vehicle to the next, West said you can use the average decreases for rule-of-thumb estimates of speed vs. fuel economy. You can assume a 12.4% decrease at 60 vs. 50 m.p.h., 14% at 70 vs. 60 m.p.h. and 15.4% more at 80 m.p.h. Thus, 41.8% lower at 80 vs. 50.

If you want to maximize your fuel economy, slow down. The numbers don’t lie.

Photo Copyright: Gergely Zsolnai/Shutterstock
  1. Lee Young 4 years ago

    I broke in the Cruze the old-fashioned way…no steady speeds/herky-jerky starts (nor stops) for the first 2K miles. granted, this probably had some impact on the DIC mpg of 25.8 which has only varied .1 over the 3 years.
    I drive by the odometer: 2,500rpm/64 mpg is the “break-point” for decent mileage on any given tank of gas. I presume that if this vehicle ACTUALLY had cruise-control, I might get another cupful of gas per tank full
    I told the salesman the car MUST have cruise control and when I looked at the DECEPTIVE “reset” marking on the left stick assumed the car had C.C. thank you G.M. (Mr.Smith is only guilty of the sin of omission)
    in wondering why some cars get a boost in mpg at higher speeds, have y’all considered the speed where the overdrive gear kicks in on 6 speed trannys; well, GM customer service told me the 6th speed is 0/D

    • Jerry 4 years ago

      Good to hear from you. I have noticed the same thing no matter whether a 6, 7, 8, or 9 Speed automatic…the fast you go, the less the miles per gallon. The effect seems to be less in the higher speed transmissions, but there is still a decrease.

      GOOD to hear from you!

      Jerry Reynolds, President
      Car Pro Radio Network

  2. […] Here is a link to an article about how much speed affects miles per gallon. Slow Down-Save Fuel – Car Pro Commentary | Car Pro USA One point it makes is if a car gets 40mpg at 50mph, it would likely get ~23mpg at […]

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