MPGs, meet GPMs

If you're looking to save gas, upgrading from a 28 mpg Ford Focus to a 46 mpg Toyota Prius is certainly a better choice than upgrading from a 15 mpg Dodge Durango to a 20 mpg Nissan Murano, right? Wrong. Over 10,000 miles, choosing the Murano-Dodge swap will save you 26 more gallons than the Prius-Focus switch.

The detailed answer as to why the Durango-to-Murano switch is a bigger gas saver puts this forum right at the heart of a fuel-efficiency labeling polemic that, if not at full, raging debate status yet, perhaps ought to be.

As any red blooded American will tell you, fuel economy is measured in miles per gallon (mpg). Unless, of course, you're in Europe, where it's measured in liters per 100 kilometers (l/100km).

To avoid any conversion migraines, not to mention the usual disdain, teeth-gnashing and downright fury provoked by all things metric, think of the European standards as gallons per mile (GPM). That's how Duke University business school professors Richard P. Larrick and Jack B. Soll did it in a fascinating recent study of the subject, called "The MPG Illusion" in the respected journal Science.

They examined how consumers fail to do the math required to figure out how much gas a particular vehicle will consume, focusing instead on the MPG number alone. The problem is that improvements in MPG have an inverse relationship to fuel savings, which is a fancy way of saying that fuel savings are dramatically reduced as you move further up the fuel economy scale.

Unfortunately, that fact escapes all but the most mathematically-inclined observers. The Toyota Prius in the above example consumes 217 gallons to go 10,000 miles, while the Focus consumes 357 -- an 18 mpg difference, or 64%, that yields 140 gallons in savings. The Murano, on the other hand, consumers 500 gallons, compared to 666 for the Durango -- a 5 mpg, or 33%, difference that yields 166 gallons in savings

But if vehicles were measured in GPMs -- expressed, say, as gallons per 1,000 miles -- the actual fuel savings between any two vehicles would be far more obvious. The Prius rates a 21, the Focus a 35.7, the Murano a 50 and the Durango a 67, and it's immediately clear that the latter switch is better than the former.

This reasoning can be helpful, for example, for families considering whether to ditch the gas guzzler or the midsized sedan in their quest to be more fuel efficient. As the authors of the study say

Reliance on GPM “nudges” people to better decisions because it does the math for them.

Perhaps the most famous (infamous?) application of this reasoning was performed by Green Car Journal, which, in a surprise decision, named the Chevrolet Tahoe Hybrid its green car of the year for 2008. The behemoth hybrid gets only 21 mpg, compared to 16 mpg for the non-hybrid version. That provoked scorn from environmentally conscious drivers who felt unimpressed by a 21 mpg rating. But using GPM, it's suddenly clear that over 10,000 miles, the hybrid saves a driver an impressive 149 gallons, or 24%.

Of course, there is a big canard buried in these kinds of comparisons: they make the assumption that the only alternative to something like the Chevy Tahoe is the Chevy Tahoe Hybrid. If the driver instead chooses a Toyota Highlander hybrid, rated at 26 mpg, the savings work out to 240 gallons, or 39%. And if they pick a Prius...

-- Ken Bensinger

Photo of 2008 Chevy Tahoe Hybrid by General Motors Corp.

Comments

[H Tran]

GPM seems to work best for improvement in driving (more efficient) and for 'guzzlers' vehicles that are bought out of necessity eg. trucks.

[Chi Wei Hu]

Sure, gallons per mile is an important measure. But I'd like to know what the difference is if you're carrying, say, an entire soccer team (which for the sake of this we'll call the Pumas) on a bus and you switch from a hybrid bus to a natural gas bus. Are the fuel savings commensurate with a switch from an Excursion to a EV1? Please explain.

Also, clearly the Big Three haven't figured out that the way to win over consumers is not styling, not fuel-economy, not great vehicle names, not great paint jobs. It's plain and simple: gadgets. Lots and lots of gadgets.

[mucker]

What you smokin man? You sure like to blow smoke. You think that because you can get a high percentage on the diff between 500 and 666 gals that that's better than only burning 217 gallons? Simple math buddy, here it is. 217 times $4.00 a gallon = $868 while $4 x 666 = $2664. That's $1796.00 less. Yike. You must be an SUV dealer.

[samuel]

well according to that logic switching from a Range Rover
to a Suburban is sublime. WTF.

[ArtInvent]

I think a more salient point is that there have been many, many drivers over the last 10-15 years who bought large V8 or V6 SUV's simply because they were fashionable and reasonably priced to own and operate. A huge number of these people could just as well drive a Prius or a Civic, and many of the rest could probably drive something the size of a RAV4 or CRV, or BMW wagon, especially if there were more upscale and stylish choices in that size range. There are thus many people who could triple and quadruple their mileage by making a smarter choice in cars, just with what's available to day. And you know what? That's exactly what's happening on dealer lots right now.

[NV]

1. you must sell SUVs for a living, to come up with this kind of logic. I'll play your game and I'll raise you one. Take this logic further and apply the same metric used by airlines, i.e mpg/passenger, and than you'll see that SUVs really make sense when fully loaded with all your friends/family. the trouble with this logic is that you need to get out and make friends !!!
2. nobody switches from a ford focus to a car twice as expensive, the Prius to save money. The Prius is for all those snooty trust fund liberals that want to see ChevronCheney squirm.
3. since when do we choose our cars because it makes sense. I dare say we make aour decision for car makes us look better, and than build our portfolio of excuses, arguments, etc

[Joel]

You're kidding with this article right? Or, as alluded to by previous posters, do you own an SUV dealership? Statistics can be made to say anything. I could give a convincing case that the sky is green or that California does not border the Pacific Ocean if the statistics are in line. But let's do the math shall we? 2008 Hyundai Accent, 39.56 mpg, fill up 9.87 gals @ $3.87/gal (I live in New Hampshire) which takes me 390.46 miles for $38.20. Any other questions? Peace

[Frank Canino]

Duke University study? Hmmm.....isn'that where the CEO of GM graduated from?

[Peter Lee]

Look, the math here is indisputably correct but it completely misses the forest for the trees. The hypothetical buyer who might be calculating MPG or GPM would presumably only have 1 car that he's thinking of retiring and he's comparing this 1 car against 2 other potential cars. The GPM comparison would be:

Car A (current auto) --> Car B (small auto) or
Car A (current auto) --> Car C (big auto)

In this case, it's virtually certain that trading his Car A for Car B is going to be a better deal, regardless of what kind of vehicle Car A is. But instead of this comparison, which tracks the choice that a real world buyer would be facing, the researchers invented this bizarre fairy tale comparison:

Car A (1st current car) --> Car B (small car) or
Car C (2nd current car) --> Car D (big car)

This is scenario, the choice that will save more gas will depend on what kind of cars Car A and Car C are. No duh. The only thing "fascinating" about this conclusion that it's fascinatingly obvious. It's also a completely invalid apples-to-oranges comparison. What buyer has TWO separate cars, BOTH of which he's thinking of trading in and he's trying to figure out which was the better trade? It's absurd. Far from helping consumers reach rational decisions, this study seems completely oblivious to how unrealistic and artificial its thought experiment is. Of course, you know this because you bury the qualifier that moots this entire posting in the last paragraph:

"Of course, there is a big canard buried in these kinds of comparisons: they make the assumption that the only alternative to something like the Chevy Tahoe is the Chevy Tahoe Hybrid. If the driver instead chooses a Toyota Highlander hybrid, rated at 26 mpg, the savings work out to 240 gallons, or 39%. And if they pick a Prius..."

[math boy]

You can fool some of the people some of the time...I'm glad the readers who posted comments were intelligent enough to see how idiotic your mathematical reasoning really is. To neglect the cost of the vehicle and gasoline is sophomoric.

This has to be one of the worst articles I have ever read. I'm surprised it was published.

It's no wonder we have a math problem in this country. Your innumeracy is contributing to it.

[pearl2k]

Fine. GPM might be easier to grasp than MPG. But the only thing the above example shows us is Durango drivers should become either Focus drivers, or better, Prius drivers. But MPG already told us that.

It also shows us that, if I'm already driving a Focus, and looking at a Prius, WTF do I care about Durango drivers?

[Rick Larrick]

The original Science article is available free (no subscription) from a link provided by Science on my research website. You can access it from the section marked “Original Article” on this webpage:

http://faculty.fuqua.duke.edu/~larrick/bio/Reshighlights.htm

The mathematically sophisticated recognize that GPM = 1/MPG. Most people, however, do not. In our studies, most people mistakenly equate MPG improvements of equal size with equal gas savings. Their reasoning is corrected when efficiency is reported as GPM.

We use comparisons such as improvements from 10 to 20 MPG versus 25 to 50 MPG as a "thought experiment" to illustrate the illusion created by MPG. People have reacted, "why not go from 10 to 50?" We agree. Everyone should go to 50 MPG.

Our central argument is that GPM is more helfpul than MPG because it clearly indicates gas savings from efficiency improvements. MPG does not.

What's the difference?

First, MPG and GPM are equally valid at telling you which car in a pair is more efficient. There is no trick or illusion there. It's a tie between the two measures.

Second, GPM provides a clearer picture of the benefits of improvements in efficiency. MPG is a misleading indicator of the amount of gas saved from a given MPG improvement. (Note that even percentage MPG improvement is not a valid indicator of total amount of gas saved.) GPM makes transparent the actual amount of gas saved for some meaningful distance. It also makes very clear the urgency in replacing the most inefficient cars.

Finally, MPG and GPM are both useful in different ways: MPG tells you the range of your gas tank; GPM tells you how much gas you’re using. We suspect that, when buying or trading in a car, most people care more about the amount of gas they will use than the range of the gas tank. GPM can be immediately translated into fuel costs and CO2 emissions. MPG cannot.

Gas savings is only one consideration in buying a car. Our argument is to equip people with a more accurate impression of gas savings.

There are two graphs on the research website posted above that show the relatinship between MPG and GPM:

http://faculty.fuqua.duke.edu/~larrick/bio/MPG%20files/GPM%20Graphs.pdf

http://faculty.fuqua.duke.edu/~larrick/bio/MPG%20files/mpggraph.ppt

When the Department of Transportation calculates CAFÉ compliance for auto makers, they take the harmonic mean of MPG for vehicles sold. The first step in calculating the harmonic mean is calculating GPM. This avoids the illusions created by averaging MPG.

[physics_phd]

Whoa. Hold on. You got a faculty job in academia and are doing research on how MPG versus GPM is more transparent. Oh, your are in the business school. The excel plots of 1/x as a function of x really illuminated the issue at hand.