Gallons per mile would help car shoppers make better decisions

Jun 19, 2008

Posting a vehicle's fuel efficiency in "gallons per mile" rather than "miles per gallon" would help consumers make better decisions about car purchases and environmental impact, researchers from Duke University's Fuqua School of Business report in the June 20 issue of Science magazine.

Inspired by debates they had while carpooling in a hybrid car, management professors Richard Larrick and Jack Soll ran a series of experiments showing that the current standard, miles per gallon or mpg, leads consumers to believe that fuel consumption is reduced at an even rate as efficiency improves. People presented with a series of car choices in which fuel efficiency was defined in miles per gallon were not able to easily identify the choice that would result in the greatest gains in fuel efficiency.

For example, most people ranked an improvement from 34 to 50 mpg as saving more gas over 10,000 miles than an improvement from 18 to 28 mpg, even though the latter saves twice as much gas. (Going from 34 to 50 mpg saves 94 gallons; but from 18 to 28 mpg saves 198 gallons).

These mistaken impressions were corrected, however, when participants were presented with fuel efficiency expressed in gallons used per 100 miles rather than mpg. Viewed this way, 18 mpg becomes 5.5 gallons per 100 miles, and 28 mpg is 3.6 gallons per 100 miles -- an $8 difference today.

"The reality that few people appreciate is that improving fuel efficiency from 10 to 20 mpg is actually a more significant savings than improving from 25 to 50 mpg for the same distance of driving," Larrick said. (See table below.)

Soll noted that replacing a large vehicle that gets 10 mpg with one that gets 20 mpg reduces gas use per 100 miles from 10 gallons to five, a 5-gallon savings. Replacing a small vehicle that gets 25 mpg with one that gets 50 mpg reduces gas use per 100 miles from 4 gallons to 2, a saving of only 2 gallons.

"Miles per gallon is misleading and can play tricks on our intuitions," Soll said.

"For families and other owners of more than one type of vehicle, the greatest fuel savings often comes from improving the efficiency of the less efficient car," Soll added. "When fuel efficiency is expressed as gallons per 100 miles, it becomes clear which combination of cars will save a family the most gas.

"We believe that everyone should try to be as fuel efficient as possible. For some people, that may mean driving the most efficient car available, such as a small hybrid car, but for others it may mean finding the most efficient option possible within their chosen class of car," Soll said. "There are significant savings to be had by improving efficiency by even two or three miles per gallon on inefficient cars, but because we communicate in miles per gallon, that savings is not immediately evident to consumers."

The authors recommend that consumer publications and car manufacturers list efficiency in terms of gallons per 10,000 miles driven, which is already the standard in many other countries. "This measure makes it easy to see how much gas one might use in a given year of driving and how much gas, and money, can be saved by opting for a car with greater efficiency," Larrick said.

Source: Duke University

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User comments : 22

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quantum_flux
5 / 5 (1) Jun 19, 2008
Well, then I would like to see a vehicle that gets 1 gallon per 100 miles then....100 miles per gallon!
RogerB34
2.7 / 5 (7) Jun 19, 2008
BS. The scheme is to condition the peasants to calculation of carbon emissions by weight. Later to tax them.
gmurphy
4.2 / 5 (5) Jun 19, 2008
I don't know whether to laugh or cry, the sales person talks about relative savings like they're better than absolute cost. Even if you save 5 gallons by switching from a 10mpg vehicle to a 20mpg vehicle, you're still using 5 gallons every 100 miles. If you switch from the 25mpg vehicle to a 50mpg vehicle you only use 2 gallons per 100 miles. This is a smokescreen by the SUV sales people, jesus tapdancing christ
Soylent
not rated yet Jun 19, 2008
Well, then I would like to see a vehicle that gets 1 gallon per 100 miles then....100 miles per gallon!


Your average scooter or moped.
Soylent
not rated yet Jun 19, 2008
I don't know whether to laugh or cry, the sales person talks about relative savings like they're better than absolute cost.


No, they're talking about absolute cost like it's better than relative savings.
jburchel
3.8 / 5 (5) Jun 19, 2008
Stupid... Either way it is the same thing and it doesn't really make any difference which way it is reported. How about freedom as a concept? Let people buy whatever they want and if they want to drive a gas guzzler, they can pay for the gas, and if they want to drive a Vespa, they can take the risk of being flattened if they collide with anything larger than a blade of grass, but save untold hundreds on fuel and their proudly miniscule carbon footprints...
Glis
5 / 5 (2) Jun 19, 2008
In case you haven't noticed, us American's aren't the greatest with fractions/decimals. It will take hours of training to get them to understand that less gallons per mile is good.

The current system is ideal for being poor. If it's 25mpg and I can afford 2 gallons of gas, I know how far I'll get without a calculator, which I don't have because I sold it for gas.
canuckit
5 / 5 (2) Jun 19, 2008
In Canada and Europe fuel consumption/efficiency of motor vehicles is measured in liters per 100 km.
Valentiinro
5 / 5 (1) Jun 19, 2008
Aptera gets 120 per gallon and seats 2.
http://www.aptera.com/
retread
4 / 5 (5) Jun 19, 2008
Please USA, go metric. In Japan, km/litre is norm, but for whatever reason, Canada has adopted the most annoying European litres/100km standard.
Also, it is true as mentioned - there's no way your gonna get 'mericans to figure out that less is more when they are so used to more being more.
joefarah
5 / 5 (1) Jun 19, 2008
In Canada we measure in litres per 100km. But ever since we've started doing this, over a quarter century ago, most drivers have lost touch with car efficiency. It's just that much more complex. Just enough to confuse most of the population. I buy a litre of gas, I want to know how far it takes me... that's intuitive. I don't drive 100km and wonder how much gas I used. That's much less intuitive.
zevkirsh
1 / 5 (1) Jun 20, 2008
no crap , everyone who knows anything about the CAFE regulation knows this.
d01phi
5 / 5 (1) Jun 20, 2008
As others mentioned, in Europe we've been using l/100km for decades. And I find it very useful for answering questions like, how much does it cost me to get from a to b. Maybe you Americans are more in the mindset of having to decide whether it is possible to cross vast deserts with a given amount of gas. But I wonder who does this very frequently, as opposed to commuting to work over a known fixed distance.
Mercury_01
3 / 5 (2) Jun 20, 2008
So are we going to switch our spedometers to hours per mile? This is dumb.
PJS
not rated yet Jun 20, 2008
Just wait until we switch to electric cars and watch the mass confusion!
DGBEACH
not rated yet Jun 20, 2008
Just wait until we switch to electric cars and watch the mass confusion!


Then it will be Amps-per-mile (kilometer)I should imagine...or better, miles-per-charge. 'Seems LESS confusing.
ErickS
not rated yet Jun 21, 2008
miles, gallons. Only Liberia, Birma and the U.S. are non-metric. Sigh.
deatopmg
1 / 5 (1) Jun 21, 2008
34 to 50 mpg may not be as big a % change as 18 to 28 mpg but at 34 mpg the owner is already way ahead of the fool switching to the 28 mpg car.

This whole study is ridiculous waste of my money.

anyway, we all (should) know that there is reduced return as we get closer to (asymptote) the limit. So the %change to 54 mpg is less but it is nearly 100% better than 28 mpg.
Bill_Hees
not rated yet Jun 23, 2008
I've been a fan of the gallons-per-mile viewpoint for several years now. What did it for me? A while back I heard a politician praise an 80 mpg experimental car and claim 'if you think 80 mpg is good, imagine how great 120 mpg will be! we've got to substantially increase funding and achieve that goal next!'. I imagined it and I realized it would save only a miniscule amount of gas beyond what 80 mpg would save, but that the bigger mpg number *sounded* so much better. I thought "great, some politician who's bad at math is going to spend our taxes in pursuit of a sound bite". So my concern is policy makers who are bad at math. This article's authors are concerned with consumers who are bad at math. Either way, same conclusion -- using "miles per gallon" hurts people's understanding of how much different levels of fuel efficiency actually matter -- or don't.

One question is what denominator is best. 1 mile? 100 miles? My personal preference would be gallons per 1000 miles. This roughly approximates gallons used per month, which puts it in the same terms as other bills and notably car payments. Then most car purchasers could more easily put a value on reduced fuel consumption.

For example, is it worth $120/month extra to get a 50 mpg Prius instead of a 34 mpg Corolla? Most people would need a calculator to figure this out. However if you're told the Prius uses 20 gallons per month and the Corolla uses 29, well, that's only a 9 gallon/month savings. Maybe you like the Prius for it's styling, but the lower fuel costs sure don't justify the higher price.

Using the right units makes everything so much clearer.
Bill_Hees
not rated yet Jun 23, 2008
> there's no way your gonna get 'mericans to figure out that less is more when they are so used to more being more.

Americans have one case of "less is more" down pretty well. We understand that lower costs are better. Just refer to gallons per mile as "fuel costs" instead of "fuel efficiency". Then less is more.
Bill_Hees
not rated yet Jun 23, 2008
> 34 to 50 mpg may not be as big a % change
> as 18 to 28 mpg but at 34 mpg the owner
> is already way ahead of the fool switching
> to the 28 mpg car.

This isn't about awarding medals to those who are "way ahead"; it's about recognizing the best opportunities for saving fuel. If you currently use only a tiny amount of fuel, by definition you cannot reduce your usage by more than a tiny amount of fuel. However someone who uses a lot of fuel has the potential to reduce their usage by a much greater amount than you can (even if after doing so they still use more than you). That's the opportunity.

Using gallons per mile instead of miles per gallon makes it more clear where the opportunities are for reducing fuel consumption.

> So the %change to 54 mpg is less but it is
> nearly 100% better than 28 mpg

Really? 54 mpg uses nearly 50% less fuel than 28 mpg. I'd call that "nearly 50% better", not "nearly 100% better".
Eco_R1
not rated yet Jun 23, 2008
one of these days fuel will be so expensive....... that i will have to push my car to the liquor store!!!