Math Is Hard: MPG Still Stupid, National Research Council Agrees

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More than a year ago, we wrote an article pointing out the flaws in the U.S. practice of measuring a car's gasoline use with the familiar miles-per-gallon (MPG) measure.

Now, no less a body than the august National Research Council has agreed with us.

As  it noted in a pre-publication summary released by the National Academy of Sciences:

Fuel economy data cause consumers to undervalue small increases (1-4 mpg) in fuel economy for vehicles in the 15-30 mpg range.

Display consumption as well as MPG

Its panel strongly urged that the information should be displayed to consumers as fuel consumed (volume of fuel per 100 miles, for example) alongside the traditional MPG measure, which is only really used in North America.

Everywhere else, car buyers assess and compare consumption, which correlates directly to their fuel costs (as well as tailpipe emissions of greenhouse gases, predominantly carbon dioxide).

So what, exactly, is the problem with good old American miles per gallon? We laid it all out more than a year ago in an article entitled Miles Per Gallon Is Just Stupid; No, Really, It Is.

It's not linear

To illustrate the problem, just answer this question: Do you save more gasoline by going from 10 to 20 mpg, or going from 33 to 50 mpg?

Most Americans pick the second choice. But that's wrong. No matter how many miles you drive, swapping out a car that gets 10 mpg for a 20-mpg version will save you five times the amount of gasoline that going from 33 to 50 mpg will. Five times!

The problem is that the MPG scale isn't linear. A 10-mpg improvement saves vastly different amounts of gasoline, depending where on the scale it falls. But fuel consumption (gallons used to go a set distance) is linear. Halve that number, use half the gasoline.

And if you still don't believe us, read the entire original article--which caused, ummmm, quite a lot of controversy when we first published it, and continues to generate readership and interest.

Sorry, folks. Math is math. Even if, to quote Barbie, "Math is hard!"

Three-year study

The NRC panel's three-year study, done at the request of the National Highway Traffic Safety Administration (NHTSA), investigated  the merits of different methods for improving gas mileage over a base 2007 vehicle.

As well as the MPG recommendation, the report concluded that a combination of existing technologies could meet the now-enacted increases in corporate average fuel economy (CAFE).

But it also noted that making cars more fuel-efficient would raise their cost, and that the payback for such measures depended entirely on the future price of gasoline.

[The Car Connection]

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Comments (8)
  1. Hi John
    I agree with you completely, a few years ago I created a spreadsheet for myself (with a little help from the web) to work out my fuel consumption.
    The first thing I did was create a miles per litre - here in the UK we've been buying (being robbed) petrol in litres for years but we're still getting MPG figures.
    Very few people here know that they're getting approx 6 miles per litre (a litre costs approx £1.20 - approx. $6.90 for a US Gallon).
    Like you I then went on to do a litres per mile which worked out at 0.17.
    I commend you on your efforts to get fuel consumption measured correctly but I fear you face an uphill battle as every oil co. and motor manufacturer wants to look better than they are and continue to con us!

  2. I also agree. The only real benefit to MPG is that it allows an at-a-glance comparison with other cars, or a consistant marker should you be following your own MPG. We use the MPG system in the UK but comparisons can also be made using the "grams per kilometer" of CO2 measurement - okay, so it doesn't tell you economy, but it's a good measure of comparison.
    In Europe, they use the much more telling liters/100km.

  3. my drive to work is 10 miles, and whether i improve my vehicle mileage from 10 to 20 mpg or 30 to 40 mpg, i get 1 extra trip to work for each gallon of fuel i purchase/consume.

  4. t_oates: you clearly did not understand the article or do the math. At 10 mpg u get 1 trip per gallon. And yes by going to 20 mpg you get to 2 trips. So you have doubled the trips per gallon. Now if you start at 30 mpg you get 3 trips per gallon and go to 40 you get 4 trips. Now you get 33% more trips per gallon. So that is 100% more (or double) and 33% more.

  5. thanks bill, it is interesting that it seems that one option appears better than another looking at %'s, but it appears either option gives me the same mechanical advantage of one extra trip? so improvements over baseline, may appear different dependent on the point of reference.

  6. With EV's the MPG and fuel consumed ratings seem to be both meaningless.
    If you have a plug in hybrid such as a Volt or plug in Prius (when they come out) just measuring a fuel use rating by how much gas goes in the vehicle is missing the amount of electricity that goes in when you charge it.
    With EV's the situation is even more so. Since you wont be putting any gas in your Leaf or Tesla Roadster.
    We really need to switch to something that expresses the true wheels to wells approach.

  7. t_oates, I think you've well demonstrated the point of the article. A 10 MPG increase no matter what starting point gives you one extra trip per gallon. Therefore, as your base mileage increases, the advantage of a 10 MPG gain actually decreases. If you drive a 10 MPG truck and swtich to a 15 MPG truck, and your neighbor drives a 40 MPG Fiesta and switches to a 50 MPG Prius, although he will use less gas, you will save far more compared to your previous vehicle.

  8. it seems that everyone is getting closer together here. brad's comment reinforces the idea that the savings of 1 extra trip per gallon are equivalent between options. now if we want to look at relative savings (%) you would be buying gallons more often with the less efficient car, but the amount of savings for each gallon purchased is the same...there is just a different starting point. and the starting point would be a fixed point-an already prchased car-per individual.

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