The Big Trouble with “Miles Per Gallon”

22821423_14e1d70f4a.jpg(Photo: snoweyes via Flickr)

Can you answer this riddle?

A town maintains a fleet of vehicles for town employee use. It has two types of vehicles. Type A gets 15 miles per gallon. Type B gets 30 miles per gallon. The town has 100 Type A vehicles and 100 Type B vehicles. Each car in the fleet is driven 10,000 miles per year. The town’s goal is to reduce gas consumption and thereby reduce harmful environmental consequences.

Choose the best plan for replacing the vehicles with corresponding hybrid models:

a. Replace the 100 vehicles that get 15 miles per gallon (mpg) with vehicles that get 20 mpg.

b. Replace the 100 vehicles that get 30 miles per gallon (mpg) with vehicles that get 40 mpg.

c. a) and b) are equivalent.

d. I don’t know.

The answer is a, but a 2008 study by two Duke University researchers found that most who answer the question incorrectly choose b. After all, a 10-mpg increase in the same number of vehicles sounds a lot better than a 5-mpg hike, right?

In fact, measuring cars’ efficiency in mpg creates what statisticians call a curvilinear relationship between fuel consumption and efficiency. So the bang for the buck that’s possible by improving mpg is largest when you’re dealing with gas-guzzling vehicles, thus making the 15 mpg cars in the riddle the best candidate for replacement.

But what about transportation pros, the types who actually measure the real-world impacts of fuel economy? One might assume that industry veterans would understand the curvilinear trick — but they still slip up in surprising numbers when asked to make efficiency decisions based on mpg, according to a new study by researchers at the University of California, Davis.

The study asked transportation pros to choose the best efficiency improvement using two metrics: one in mpg and one in “gpm,” which restates fuel economy based on how many gallons a car burns every 100 miles. Nearly 40 percent of the industry respondents answered b when asked that tricky riddle, which the U.C. Davis team concluded “raise[s] questions about the policy and regulatory implications of representing fuel economy in mpg to make decisions about fleet efficiency.”

Still, that’s not the only negative consequence of relying on mpg to measure efficiency. Car companies are currently fined $55 per mile for violating federal CAFE standards (a charge that was last increased in 1997) — but those fees are applied as if all consumption violations are equal, when a heavy truck breaking the rules is more of a drag on efficiency than a passenger car in violation.

If CAFE fines were imposed on the auto industry using gpm, not mpg, “truck violations would be penalized more and many passenger car violations would be penalized less,” the U.C. Davis researchers concluded.

What’s more, the government would actually be collecting more from the most egregious CAFE rule-breakers if gpm were used. The U.C. Davis team estimated that about 20 percent of current penalty burden would be redistributed, largely hitting trucks and less efficient vehicles.

The U.C. Davis researchers behind the report are Dana Rowan, Alex Karner, and Deb Niemeier. “At a minimum, future rules should be written to prevent MPG-based distortions,” they conclude — a nuance that was lost on the authors of the recent “cash for clunkers” rebate program.

  • Isn’t D a technically correct answer too?

  • David Galvan

    We should just invert the conventional standard and talk in terms of gallons-per-mile. Invert the inverse relationship and the numbers become much more intuitive for comparisons.

  • Canada uses litres per 100 km, which eliminates a lot of the distortions that you see with MPG.

  • DJB

    Yeah, liters/100km is better, for the reasons described in the main post, and because it’s metric. We should stimulate our economy by switching over to metric like the rest of the world (lots of jobs putting up new road signs) :)

  • DJB

    On the other hand, I’d rather that people think about MPG, than not at all about fuel economy, which I think is probably more typical of Americans, given the cars they drive. Maybe switching to another measure, despite its merits, would have the disadvantage of confusing people and discouraging them from thinking about fuel economy.

  • DJB,

    My god, sometimes I feel like I’m the only person in the whole damn country who wants to get us off our primitive system of feet and miles and go metric. Glad to know there’s someone else out there. I believe we tried to adopt it in the 70s, but then Reagan in his infinite wisdom decided that the metric system was an example of evil government intervention and cut spending for the US metric program. Today, anyone who even brings up the metric system is considered “weak on terror” or “French” – looks like our system of weights and measures will be forever stuck in the stone age.

  • Metric makes so much more sense than our current system. Science tells us this.

    But it is the fear of doing something different which scares people irrationaly.

  • peteathome

    I had a similar go around with a bunch of eco bike activists a couple of years ago. I was arguing that car pooling was probably the fastest approach to reducing gasoline usage and carbon emissions.

    The example I used was getting half the drivers to carpool with the other half. This would half the effective gas usage ( or double the miles per gallon per passenger), exactly the same as if half the drivers switched to bicycling.

    I said upfront we were to ignore the effects or driving to pick up the passenger, etc.

    It was just a simple thought experiment- since I thought we would be more likely to get more people to car pool than to switch to bicycling, this would be the more effective approach if just one was to be focused on. If we could get the average car to carry a total of 3 people, that was like removing 2/3rd of the cars from the road or increasing the effective gas mileage 3x.

    But I hit a stone wall of understanding with them. They kept going that bikes consumed no fuel while cars did, so bikes were better.
    They literally could not understand the argument – since cars always use fuels and bike don’t, they thought we would use less gasoline if some drivers switched to bikes rather than car pool.

  • DJB

    Carpooling is definitely an important strategy to reduce the use of fuel and vehicle infrastructure. I don’t think it’s necessary to portray it as carpooling versus cycling though. For a 50km trip carpooling might make more sense. For a 5km trip cycling might make more sense.

  • peteathome

    It was either/or but which of the two approaches would have the biggest impact, soonest.

    They got confused by the question, “which saves more fuel”:

    doubling the mph of all the cars;
    keeping half the cars the same and replacing half with “cars” that have infinite mph.

    In the real world, it’s more interesting. Imagine half the commuters have trips of 20 miles and half have trips of 5 miles, just to keep it simple.

    Which saves more fuel? Getting all the 5 mile commuters to switch to bikes or getting all the 20 milers to carpool?

    Obviously, doing both is best, but if you could just do one? (or maybe, which is more likely to happen ;-)

  • peteathome

    Make that “wasn’t either/or”


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