Buy an S.U.V., Save the Planet


Scientists and engineers are racing to develop technologies that will improve fuel economy and perhaps replace gasoline altogether. This is certainly to be applauded. But there may be an easier and more effective way to help wean ourselves off foreign oil and fight global warming. Interestingly, it involves not 21st-century technology but 28th-century technology — as in 28th-century B.C.E.

“Thanks to the mileage mirage, our efforts as a society may be somewhat misplaced.”

What’s better, it will enable us to shed the pounds with comparatively little diet or exercise. We can improve fuel economy not through the onerous task of developing next-generation lithium-ion batteries but simply by getting people behind the wheels of S.U.V.’s. How?

We currently measure fuel economy in miles per gallon (m.p.g.), a number which is helpfully plastered in the window of every new car. However, less helpfully, m.p.g. is a very deceptive way of measuring fuel consumption. Here’s how it tricks us.

Consider the 2009 Toyota Prius, poster child for the environmental movement. The U.S.E.P.A. estimates the car gets an eye-popping combined highway/city 46 m.p.g. This is certainly an impressive achievement.

Compare this to one of those sinister S.U.V.’s that we greens love to hate, the 2009 Toyota RAV4 2WD. The Toyota is one of the more fuel-efficient non-hybrid S.U.V.’s on the market, but its combined m.p.g. is 24, which is well below the Prius’ 46. Ignoring the rebound effect (which I blogged about here), in a typical year (say, 12,500 miles of driving) the Prius uses 250 gallons less than the RAV4.

Now compare the RAV4 to the Land Rover Range Rover Sport. At 14 m.p.g., the Range Rover is a gas guzzler even by sport-ute standards. Let’s say we swap it for a RAV4. This would improve our m.p.g. from 14 to 24. This is an improvement, but it would certainly save less fuel than a switch from the RAV4 to the Prius, right?

Actually, no. In fact, upgrading the inefficient S.U.V. to a more efficient one would save a lot more fuel — 372 gallons per year — than the 250 gallons saved from the switch from an efficient S.U.V. to the most fuel-efficient car on the market.

Why does 10 m.p.g. matter more than 22? The reason is that the relationship between m.p.g and fuel savings is not linear but curvilinear. Ten m.p.g. at the bottom of the range matters a lot more than 22 m.p.g. higher up.

This is a hard concept for us to get our brains around. Richard B. Larrick and Jack B. Soll, reporting in Science (gated) found that only 1 percent of college students studied correctly perceived that an improvement from 14 to 24 m.p.g. saves considerably more fuel than an improvement from 24 to 46.

To give our brains a break, we might adopt a better way to look at fuel efficiency, aided by the manipulation of a mathematical tool in use in the Indus Valley almost 5,000 years ago — the unglamorous fraction.

The trick is one that even fourth-graders can master: invert the fraction. Let’s consider not miles per gallon but gallons per mile (or, to make the numbers prettier, gallons per hundred miles). By this metric, we get an unclouded picture: the Prius uses 2.17 gallons per hundred miles, the RAV4 uses 4.17, and the Range Rover uses 7.14.

Thanks to the mileage mirage, our efforts as a society may be somewhat misplaced. There are plenty of policy ideas afoot to get people into state-of-the-art, fuel-efficient cars, but a lot less interest in simply getting people out of the worst gas guzzlers into moderately more efficient alternatives, even within the same fuel-hungry class.

Yet a focus on the bottom would certainly be more practical. Discouraging people from buying the worst fuel hogs could be pursued not by complex technological breakthroughs but by humble tax instruments like raising fuel taxes (which I blogged about here) or revisiting Section 179 of the tax code, the so-called “Hummer tax loophole,” which actually gives tax breaks for the purchase of large, uneconomical vehicles. (Unfortunately, it was extended as a result of the stimulus legislation.) It would make sense for the government to change mileage mandates from m.p.g. standards to gallons-per-100-miles standards as well.

Even better, how about a policy solution that is almost completely painless? Let’s simply show car buyers (and the rest of us) the g.p.100m. figures instead of the m.p.g. To be fair, car window stickers currently show the annual estimated fuel cost. But the m.p.g figure takes center stage, much as it does throughout our society. Changing window stickers and the voiceovers in some car ads seems a lot easier than developing new generations of ultra-lightweight car body materials, and it could have a significant impact.

Granted, it would take some getting used to the new metric. But surely Americans can eagerly and enthusiastically adopt a new measurement system. Particularly when it comes to saving the planet, I’m confident we’ll be willing to go the extra 1.60934 kilometers. (Hat tip: Brian Taylor)

Related: The Wheels blog discussed m.p.g. math here.

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  1. Philip says:

    In Canada we use a litres/100km metric, much like your proposed gallons/100miles.

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  2. Brian says:

    I’m not certain, but I’m pretty sure that many countries in Europe use an inverted metric, which you point out makes so much more sense.

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  3. Rob says:

    You draw a hilarious conclusion out of your apples-for-oranges comparison. Howabout getting people in stupidly badly made SUVs into a Prius? Then we’d save even more (622 gallons per year, according to your figures, instead of just 372).

    Your g.p.m. recommendation seems obtuse as well: it doesn’t convey curvilinear connotations any more the m.p.g. does.

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  4. Sagrilarus says:

    About that elephant in the room . . .

    No consideration of moving from the Range Rover to the Prius or another more fuel-efficient vehicle? Your discussion of moving only as far as the Rav4 kind of misses the point, especially since the amount of space in each (and very likely intended usage) is pretty much the same.

    Anyone not bright enough to figure out the fraction one way isn’t going to figure it out in the other.

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    • Ryan says:

      Untrue, flipping the fraction linearizes the relationship. So there is no effort in the interpretation of GPM. And for a blog about economics few people tend to realize that the point here is about equating the marginals.

      If we assume that the consumer has done so, and chooses the worst fuel economy, we can make the consumer, and everyone else better off by substituting to a similar vehicle. I believe that is the point behind not switching directly to the Prius.

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  5. Gale Thorne says:

    Excellent point. America isn’t generally a very math literate place (what portion of those buying homes actually understood the terms of those loans, and what that meant for them financially?) An intuitive presentation that doesn’t require further calculations to understand the actual financial impact would have amazing results.

    Where’s the petition?

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  6. Casey Barker says:

    Thanks for pointing this out — it’s a pet peeve of mine. MPG is a useful number for computing the range of your tank, or how far you can get on the money in your pocket, but it’s not very helpful when comparing the economic and environmental aspects of vehicles.

    If we’re picking a new metric, I’d suggest “Gallons Per 1000 Miles,” as the results are generally friendly double-digit values and carry enough significance that you can safely round the decimal points off. It also gives a pretty good estimate of monthly fuel consumption for average (12,000 mile per year) drivers.

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  7. Sarah says:

    I thought you were going to suggest that the SUV could be more efficient if it actually carried the promised 7 passengers on most of its trips. Buying an enormous SUV that can carry a posse of passengers is a little less ridiculous if it’s used for heavy-duty carpooling. Unfortunately, few — of any type of vehicle — are.

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  8. odograph says:

    I’ve owned SUVs in the past, I own a Prius now.

    I know someone switching this month from a Ford Expedition to a Prius … how’s that for a jump!

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