Will Grading Cars Dispell or Enhance “MPG Illusion”?

by Marlo Lewis on September 2, 2010

in Blog

As discussed in my recent post “Obama’s EPA: School Marms R Us,” EPA and the National Highway Traffic Safety Administration (NTSHA) are proposing to revise the mandatory fuel economy label or “sticker” affixed to new cars to include letter grades based on the car’s fuel economy and carbon dioxide (CO2) emissions. Electric vehicles and plug-in hybrids would get an A+; the biggest, heaviest, gas guzzling SUVs would get a D.

To view the current sticker, click here. To see what the tut-tutting scolds at EPA and NHTSA want to replace it with, click here.

 Among other rationales for the new sticker design, the agencies claim that adding letter grades will help consumers make smarter purchases by combating something called the “MPG Illusion.”

The MPG Illusion refers to the common misperception that fuel savings from mpg increases are linear. People often assume that each additional 1 mile per gallon increase in a vehicle’s fuel economy reduces fuel consumption and gasoline expenditures by the same amount. Hence, some may conclude, if they can’t afford (or simply don’t want) a Toyota Prius, Chevy Volt, or some other high-mpg vehicle, there’s no point in buying a car with only modestly better fuel economy than their current vehicle. In reality, fuel consumption avoided and dollars saved decrease as mpg increases. Which is to say, the biggest fuel savings come from modest fuel-economy improvements in the lowest mpg vehicles. Some hypothetical (indeed fanciful) examples will make this crystal clear.

Suppose that your current car gets only 1 mile per gallon, you drive 100 miles per week, and gasoline costs $3.00 per gallon. This means you consume 100 gallons and spend $300.00 per week. If you replace that car with a 2 mpg vehicle, you’ll consume 50 gallons and save $150.00 per week. At the very bottom end of the scale, even a 1 mpg increase in fuel economy yields big savings.

Suppose now that your current car gets 99 mpg, you drive 100 miles per week, and gas costs $3.00. This means you consume 1.01 gallons and spend $3.03 per week. If you replace that car with a 100 mpg vehicle, you’ll consume 1 gallon and save 3 cents per week. At the very top of the fuel economy scale, the fuel and cost savings from an extra 1 mpg are negligible.

Turning to more realistic examples, EPA and NTSHA calculate (p. 28) that replacing a 10 mpg vehicle with a 15 mgp vehicle saves 33 gallons of gas for every 1000 miles driven whereas replacing a 30 mpg vehicle with a 35 mpg vehicle saves only an additional 5 gallons of gas for every 1000 miles driven. The same increase in fuel economy — in this case, an extra 5 mpg – saves more than six times as much fuel if the vehicle replaced gets 10 mpg rather than 30 mpg.

Professors Rick Larrick and Jack Soll of Princeton University put the MPG Illusion on the map when they published an article about it in Science magazine. They clearly explain the basic arithmetic in this Youtube video. Their illustrative case assumes a motorist who drives 100 miles per week. If the motorist has a 10 mpg vehicle and switches to a 20 mpg vehicle, he’ll cut his weekly fuel consumption from 10 gallons to 5 gallons — a savings of 5 gallons. If the motorist has a 25 mpg vehicle and switches to a 50 mpg  vehicle, he’ll cut his weekly fuel consumption from 4 gallons to 2 gallons — a savings of only 2 gallons.

“The key insight,” says Larrick, “is that improving inefficient cars, that have low mpgs, by even low mpg increases, saves a lot of gas.” Soll elaborates: “If you’re comparing two vehicles, one that gets 12 miles per gallon and the other that gets 15 miles per gallon, if you drive 10,000 miles in a year, you’ve saved about 170 gallons of gas [in the 15 mpg vehicle], and that comes out to be about $700.00 at $4.00 a gallon. So this [savings] is a significant amount even though the jump from 12 to 15 [mpg] may look pretty small.”

To counter the MPG Illusion, Larrick and Soll advise policymakers to express fuel economy in terms of the amount of fuel consumed per unit of distance traveled. Expressing fuel economy in the conventional way, as miles per gallon, leads people to “undervalue small improvements on inefficient vehicles” and “underestimate the value of removing the most fuel inefficient vehicles,” the researchers argue in Science magazine.

This, of course, is music to the ears of the anti-SUV crowd. Greenies would love to believe that the market for SUVs is sustained by an “illusion.” Because if that is so, then EPA and NHTSA can depress SUV sales just by making simple changes in how fuel-economy information is presented — just by redesigning the sticker

Years of SUV-bashing, fuel-economy prosyletizing, climate-change scaremongering, and high gasoline prices have failed to kill SUV sales. Could that have something to do with the attributes of the vehicles — their size, safety, and utility? I mean, there are objective differences between SUVs and cars greenies insist are “smart.” Just have a look! Nothing illusory about that.

If the MPG Illusion has anything to do with SUV sales, then you gotta ask: Who’s responsible for foisting the illusion on the public? Answer: the very people who’ve tried to brow beat us into believing that the only vehicle attribute worth considering is its mpg — the preachers and proselytizers of fuel economy! There’s no escaping the law of unintended consequences.

EPA and NHTSA  propose to combat the MPG Illusion in two ways. First, the sticker will estimate how many gallons of fuel the car will consume per 100 miles (as per Larrick and Soll’s advice). Second, the sticker will carry a letter grade. Presumably (the agencies don’t spell it out), EPA and NHTSA expect that bad grades will stigmatize gas guzzlers and discourage people from buying them.

Although the first option may counteract the MPG Illusion, the second will enhance it. As Larrick and Soll show, there is only a small difference in fuel savings between a 25 mpg car and a 50 mpg car. However, in the proposed EPA/NHTSA ratings (p. 37), the 25 mpg car gets a B and the 50 mpg car gets an A-. As anyone knows who has ever applied to college, an A- GPA is way better than a B GPA. The grading system implies that the biggest fuel savings are achieved at the top end of the scale.

On the other hand, a 14 mpg vehicle gets a C- whereas a 17 mpg vehicle gets a C. That 3 mpg increment is a big deal in fuel savings, according to Larrick and Soll. Yet how many car buyers will be impressed because a particular vehicle is rated C rather than C-? Except in jest, I’ve never met anyone who boasted of getting solid Cs in high school or college.

In short, the proposed EPA/NHTSA grading system perpetuates the MPG Illusion, which, unfortunately for fuel-economy zealots, cuts both ways. The illusion of linearity not only under-values savings from fuel-economy improvements in low-mpg vehicles, it also over-values savings from fuel-economy improvements in high-mpg vehicles.

EPA and NHTSA, apparently, want to manipulate the MPG Illusion rather than actually dispell it. They don’t like the illusion when (as they believe) it promotes SUV sales, but they like it when (as they hope) it promotes hybrid, plug-in hybrid, and electric vehicle sales. But the attempted manipulation fails, because the grading system, like the MPG Illusion, both over-values high-end mpg improvements and under-values low-end mpg improvements.

Grading cars actually means grading the people who buy them. People who buy cars with super-low or zero emissions are A or A+ people. Those who buy gas guzzlers wear dunce caps. The South Park spoof on the “Toyonda Pius,” Smug Alert, all-too-accurately depicts the greener-than-thou pretension of EPA and NHTSA’s proposed grading system.

Tom September 2, 2010 at 5:47 pm

I have always found fuel per unit distance far more meaningful myself, as it happens, rather than its reciprocal. I was surprised not to see the word reciprocal mentioned here.

I think you could have saved a thousand words with a picture here! Graphs don’t frighten people that much, you know! At least, not the way wordy arithmetic does.

Good article though. Good point. The difference between an A and a B matters more to most people than a B versus a C. I also notice that the distance per unit mile still appears only once on the proposed label, as compared with three instances of MPG, and doesn’t give separate figures for cities and highways.

When technology improves, will there be A**s and A***s? Clearly, this sequence is backwards. Rather than school grades they should make it analogous to cup sizes of bras.

You know it makes sense.

Tom September 2, 2010 at 5:48 pm

A++ and A+++, I should say. As ever, I was confused by having an English education.

You know what I mean though.

Comments on this entry are closed.

Previous post:

Next post: