MPG vs GMP

July 24, 2008 at 8:21 am | | science and the public

In a recent “Policy Forum” article in Science, a business professor from Duke argued that we should start reporting gallons per mile instead of MPG. The reason is that fuel used is a linear function of GPM, but an inverse function of MPG. Since humans think best in linear relationships, GPM is the most straight-forward measure to report fuel efficiency.

The real problem with MPG is that the same change in the MPG correspond to a huge change in fuel used at the low MPG end, and almost no change if a car already has a high MPG rating. Going from 20-25 MPG saves more gas that going from 35-50 MPG; going from 12-14 MPG saves more than either. This isn’t intuitive, and you really need to calculate the savings per mile in order to make a rational decision.

The take-away message is that we can’t do calculations like (1/a – 1/b) in our heads.

8 Comments »

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  1. This would be an even more important consideration in selecting vehicles for a fleet. And I’m guessing that in many cases, vehicles that get low gas mileage may also have higher emissions.

    Also in Science (30 May) was a report that claims humans innately intuit that numbers map onto space using a logarithmic scale, but many or most cultures cause this conception to shift to a linear scale. (Dehaene &al., p. 1217-1220)

    Comment by mark — July 27, 2008 #

  2. I didn’t know that in USA is in use such “masochistic” unit (and at this points I would like to now more about the reasosn which generated such decision). What the professor pointed out is exactly the normal unit used in Europe: litre/100Km (using SI obviously).
    Using just liter/Km could generate numbers like 0.081287 liter/Km but using as unit liter/100Km allow to reduce the rounding on decimals reading it as 8.1 liter/100Km

    Comment by MaurizioN — July 28, 2008 #

  3. Neat post, Sam. Thanks. While we’re at it we might as well switch to liters per kilometer.

    And I wonder how much of this flip-thinking would have helped me out in math — like square inch per parts as opposed to parts per square inch, seconds per meter as opposed to meters per second, squared seconds per meter as opposed to meters per second squared, one boy for every two girls as opposed to two girls for every boy. Wait…what?

    Comment by jordan — July 28, 2008 #

  4. Well, I can kinda see why it used to be MPG. Back in the day, we wanted to know how far, say, a 6-gallon tank of gas would take us: 30 MPG will take us about 180 miles before we refill, right?

    Now that cost is such a raging issue, the flipped equation is becoming more useful. Rather than caring how many miles we can go on one tank, we care about how many tanks we’ll need to buy in order to go some number of miles.

    In the future, pollution might trump cost as the big issue of the day, and we’ll shift again toward another equation: miles per grams of CO2 or grams of C02 per mile.

    Jordan

    Comment by jordan — July 28, 2008 #

  5. Good try with the link to Duke.

    Comment by wood — August 1, 2008 #

  6. yeah, duke sucks. ;)

    Comment by sam — August 2, 2008 #

  7. […] can see more on this at the Everyday Scientist, who says: he real problem with MPG is that the same change in the MPG correspond to a huge change […]

    Pingback by Myth: Doubling your mileage will save you lots of money (OR news from geek dad) « Sciencegeekgirl — August 16, 2008 #

  8. […] was a piece in Science magazine about how we should all be using GPM instead of MPG. Here’s a link to my post about it back then. The main point is that the relevant unit for fuel use should be fuel […]

    Pingback by Everyday Scientist » are fuel economy gauges doing math wrong? — March 26, 2013 #

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