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June 26, 2008

Gallons per mile?

Posted: 02:40 PM ET

It may sound backwards, but that is how two Duke professors suggest we gauge fuel economy.

MPG to GPM Conversion Chart

Richard Larrick and Jack Soll of Duke University's Fuqua School of Business say the phrase ‘miles per gallon' misleads consumers.

The premise of their study, published recently in the journal Science, is that you save more gas by switching from a 10 to a 15 MPG car than by trading in your 25 MPG ride for, say, a 50 MPG Prius.

If you're scratching your head, consider the following equation:

Driving a 10 MPG SUV for 100 miles requires 10 gallons of gas, while one that gets 15 MPG will burn 6.7 gallons of gas on the same trip. So the jump from 10 to 15 MPG is a savings of 3.3 gallons of gas.

On the other hand, the 25 and 50 MPG cars burn 4 and 2 gallons, respectively, on a 100-mile drive. That is a savings of only 2 gallons compared with the SUV driver's 3.3 gallons. You see?

So when it comes to saving money, Larrick and Soll say it's less about buying the most fuel-efficient car, and more about removing the most inefficient vehicles from the road. They recommend fuel efficiency be displayed as ‘gallons per 100 miles' (GPM) instead of the traditional MPG.

That way, instead of aiming for a car with the highest MPG, consumers would be striving for the lowest GPM rating.

The point is to help consumers easily understand the amount of gasoline they will save when they trade in a car.

It's simple math. To calculate gallons per 100 miles, simply divide 100 miles by the vehicle's MPG rating. However, this calculation is not one consumers seem to do when considering a new car.

For example, Larrick and Soll asked participants in their study to decide whether replacing 15 MPG vehicles with 19 MPG ones was better or worse than exchanging 34 MPG vehicles for those with 44 MPG ratings.

Groups given the vehicles fuel efficiencies in MPG chose the wrong answer of 34 to 44 MPG 75% of the time. On the other hand, people given fuel efficiency choices in GPM made the wrong decision only 36% of the time.

So yes, it's still true that driving vehicles with the highest fuel efficiency possible is still best for the environment, but as gas prices rise will consumers be making their trade decisions for Mother Nature or their wallets? If it's the latter, they may need GPM instead of MPG to make the best choice.

Test your MPG understanding at:

- Julia Griffin, CNN Science & Technology

Filed under: Cars • economy • environment • Fuel • Gas • Gasoline • Uncategorized

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Larian LeQuella   June 26th, 2008 2:46 pm ET

Wait, using math and science will actually benefit people? What ever will the marketing people do now if they can't trick us into buying things we don't need to buy?

(/sarcasm off)

Jeff   June 26th, 2008 3:39 pm ET

I say that this is retarded. I see their point, but its not that meaningful and its also not what people are used to. It would just cause confusion and make it harder for people to compare to older cars. This is just two guys trying to be clever, when really it amounts to nothing.

Franko   June 26th, 2008 4:01 pm ET

Get metric,
Reduce effort, crashes in space,
USA will decline to British way of life via the Slug under your Foot system.

J. A. Oliver   June 26th, 2008 5:00 pm ET

This concept for evaluating fuel economy may be "new" in the U.S.; but on European vehicle spec sheets, the listing has been in liters per 100 kilometers (which is the same idea) for as long as I can remember.

catlanta   June 26th, 2008 5:30 pm ET

just how stupid are these people and why would we waste our time and energy to change the way the wording is when it always has been mpg othr then gpm sounds more negative everyone should know 5 mpg savings is less then 10 mpg savings if anything the government should have stricter enforcement of how many mpg a vehicle should get to be allowed to stay on the road unless for comercial use

CB_Brooklyn   June 26th, 2008 6:10 pm ET

Surprise Surprise...

First, watch this Reuters news clip:

Then, read this article:

Phil   June 26th, 2008 7:07 pm ET

Just what I was thinking. Europe has been doing this for years, just in metric. Nothing really new.

Jim   June 26th, 2008 11:03 pm ET

That's all well and good, but the true and more useful way to look at it would be in cents per mile. All too often we gripe and moan about the cost of fuel when we go to fill up our tanks, but there should be an immediate recognition of what we pay. With the ever-increasing cost of fuel we should recognize that the daily cost of driving is changing. At 4.79 per gallon today where I live, and the mpg of my pickup, each time I drive the three miles back and forth to work it costs me $1.80 each day. Cost per gallon divided by miles per gallon times miles driven equals cost per mile.

Snuffy   June 27th, 2008 2:40 am ET

screw the mpg or gpm. Where are the 60 mpg cars that were around in the late 70's during the "Gas crisis"? Don't give me garbage about increased emission standards....BS...
We need electric cars like the one in Calif that gets over 200miles on $4.00 worth of electricity and then lots of solar cells on ALL our roofs to cut the whole country's electric bill and save the planet. The volume of homes and businesses with them would bring the cost down and help everyone...THATS what we need to do to fix the problem. Then Exxon, Shell, Mobile and OPEC could go ****** themselves...

Franko   June 27th, 2008 2:47 am ET

Drink a liter of beer ? Not before you drive.
Drink a slug of beer. What is that ? The size of a snail without a shell ?

When NASA engineers drink, under the British System; Later try to think scientific, there is confusion, as exemplified by the Mars robot soiling itself.

Under which system did Einstein drink, Wernher Von Braun ?

Dave K   June 27th, 2008 8:35 am ET

The real significance of this issue is when you look at government fuel economy requirements in the US. The CAFE law specifies minimum fleet MPG.

It's true that GPM (or liters per 100 Km) provides more insight into fuel economy. However, it's also true that switching the mileage ratings in the US would be difficult to do (inertia), make comparisons more difficult for a while, and it wouldn't change behavior very much.

We could significantly change the behavior of the vehicle manufacturers by changing CAFE to an average GPM rather than average MPG. This would not be nearly as difficult as changing to GPM for consumers, and it would yield much greater results.

CB_Brooklyn   June 27th, 2008 9:08 am ET

Instead of beer, drink tea. That way you can refuel the car when needed!! (What the Reuters news clip I linked above to understand...)

Doc   June 27th, 2008 9:17 am ET

As someone currently involved in a research study on fuel economy and corporate auto fleets, I can say the difference between mpg and gpm is huge when multiplied. Some may say the difference seems trivial, but when multiplied by the thousands of miles you drive per year, or the hundreds of cars in a large corporate fleet, or the millions of cars on the American roads, the difference adds up.

There are also major policy implications. Our government is pushing for super-efficient vehicles and making pie-in-the-sky promises about energy independence. But, those policies come with high prices and often rely on still undeveloped technology. Using gpm shows us we're better off focusing on getting the worst vehicles off the road rather than trying to get a 33 mpg car to get 50 mpg.

As a side note, since this article was about math, did anyone else notice the math error? 10 gallons minus 6.7 gallons is 3.3 gallons, not 3.4.

Parshant Kumar   June 27th, 2008 11:21 am ET

Dependence on oil is so that US economy which is the world leading economy seems so fragile or it is just speculation of price guaging on statment of strike in Nigeria, or rumors of strike on Iran. Everytime there is news on threat on oil producing country , price in US go up and when there is news about increse oil prodcution then still it rises. It seems that there is nothing much is being moniotred by Fed/State to control the threat or production increase. To me, it is like ever incresing price, irrespective of any news.
Is it totaly ignoring civics or business??

Joe from florida   June 27th, 2008 1:23 pm ET

After spending a fair amount of time in Europe it astounds me how fortunate we Americans have been. Gas and food and fuel in the US are Better and cheaper than Europeans citizen's realize.
Now the strategy for lowering gas prices in America.
1. Reduce the speed limit to 45 MPH. Gas savings here
2. Heavily promote scooters, cycles and bicycles as alternatives to gas guzzlers – Reduced gas consumption here.
3. US automakers focus on High mpg vehicles already being produced. Gas savings here too
New industries will flourish, big auto will adjust quickly and should fall in line. All this could be done at the drop of a hat. What say AC ?

michael   June 27th, 2008 2:13 pm ET

This is just a stupid way of confusing the math so a professor can publish a paper. It doesn't matter how much gas I'm burning when I really can't afford to drive in the first place.

Dave K   June 27th, 2008 2:49 pm ET

As a follow-up to my earlier post, Prof. Larrick pointed out that the CAFE standards use the harmonic mean. Because of this, CAFE effectively already uses GPM rather than MPG in its formula.

John S   June 27th, 2008 3:40 pm ET

Just about every country except USA had been using this "super new concept" for decades now.

Pete Davis   June 27th, 2008 4:26 pm ET

Fuel efficiency has been described as litres/100km here in Canada for as long as I can remember. One day, sooner rather than later, the U.S. should get with the program!

John   June 27th, 2008 5:43 pm ET

It would be a better article if the mechanical pen edited his MPG and GPM before publishing as it is written makes no sense just nonsense.

DNA   June 27th, 2008 6:10 pm ET

The problem with this "algorithm" is that people don't have a choice about which car they are trading from. It's great to know that switching a 15MPG car for a 19MPG car will save you more fuel than switching a 34MPG car for a 44MPG car, but if all I have is a 34MPG car, then how the hell does this help me?
If you're buying a new car, try to get the best MPG you can find, it doesn't need to be any more complicated than that.

DNA   June 27th, 2008 6:12 pm ET

Also, I forgot to mention that trading a 15MPG car for a 44MPG car will save you the most fuel. I don't think professor smarty-pants mentioned that.

DNA   June 27th, 2008 6:13 pm ET

If the oil industry had not killed mass transit in the sixties, we wouldn't be in this mess.

WAYNE   June 27th, 2008 7:32 pm ET

15MPG ** 6.7 **
25.MPG ** 4.0 **
50MPG ** 2..0 **

Larry in Harpers Ferry,WV   June 28th, 2008 10:57 am ET

This is the most ridiculous thing I have ever heard.Exchange the 15 mpg vehicle for the 44 mpg one.This sounds like something dreamed up by these scientists who have been paid off by the oil/automobile companies(they are essentially the same people) to try and sell more gigantic 19 mpg SUV's.Most of the people who drive these things have never seen snow in thier life and if they did would slide sideways because they have no idea on how to drive them correctly anyway.By the way I drive a 1986 Bmw 535i that gets over 30 mpg (manual transmission) and has over 300,000 miles on it.More people should choose manual transmissions because they last longer,get better mileage and you can coast down large hills which saves alot of fuel

Bill from Nebraska   June 29th, 2008 12:43 am ET

Alot of People are tradeing in their nice roomy comfortable cars that gets 22 mpg on average ,,,,for what? A compact,, less head & leg room and an uncomfortable ride ....The hybreds are nice to comute in but the average Hybred gives you approx 15% more mpg....Now lets figure: your savings in mpg for 8 – 10 years sounds good,,,,howerver the electric motor batteries last for 8-10 years....The cost for the average Hybred replacement electric motor battery cost on the average of $3,000.00 plus not counting installation charge....Now do you think they are really saving money in the long run on fuel? !!!!! Another thing is that hybreds deapreciate greater than conventional gas cars,,,mainly due to the wear & tear of the electric motor, related electronic circuits plus the battery life left...Eventually the price per Gal. will drop in the future......

Patrick M   June 29th, 2008 6:44 am ET

The writer of this article has tried to be cute by picking the most outrageous quote from a scientific paper. The real point of the article is buried in the middle, "...removing the most inefficient vehicles from the road". This is exactly analogous to removing the most polluting vehicles from the road. Putting 10 extremely clean cars on the road in place of 10 fairly clean cars does not remove as much pollution as replacing one really bad car with a mediocre one. If you don't have a 10 mpg car to start with, nobody should suggest you get one to replace it with a 15 mpg vehicle. Whatever you have, you should consider getting a more fuel efficient vehicle. Don't aim for an incremental gain, get the maximum that is available (that still suits your needs, don't try to cram 6 children in a Prius). Also, don't just consider the current price of fuel, but extrapolate what it will be for the entire life of the vehicle. Don't forget to consider resale value in your calculations. You can recover some of your investment when you sell your vehicle, over and above your fuel savings, but if you choose a car with for example a 30 mpg efficiency, in five years that may not be considered very good mileage and you will not have a good resale price, thereby skewing your overall costs into the negative.

Patrick M   June 29th, 2008 6:57 am ET

This article is primarily for people with more than one vehicle. To put it most succinctly, "replace the lowest mileage vehicle first". Another consideration is that if one vehicle is driven far more than another it should be replaced first. Calculate the number of gallons of fuel used by each possible combination in one year and pick the lowest.

Adam   June 29th, 2008 12:31 pm ET

This is dumb math! The person who traded up from 10 to 15 mpg may have trimmed 3.3 gallons off of thier consumption but is still buring almost 5 gallons ($20) more to drive 100 miles than the person who traded up from 25 to 50 mpg who trimmed only 2 gallons off of thier consumption. I guess if you are the only person sitting in that big 7-passenger vehicle this is the kind of logic that is supposed to make you feel like not such an idiot.

=DSA.   June 29th, 2008 8:17 pm ET

Wayne and Adam are about the only two who seem to get it: THE MATH IS IRRELEVANT AND POINTLESS. What has been demonstrated here is really just a percentage calculation. But when the goal is to reduce overall fuel consumption, calculating a percent savings isn't meaningful if the bottom line isn't the lowest number (of gallons used). If you drive the 15mpg vehicle, you're still using over three times the fuel to go the same distance (as the 50mpg vehicle).

And I thought "Idiocracy" was just a scifi comedy...

Franko   June 30th, 2008 7:01 am ET

The Dodo died out.
When the end of the Slug Foot Second system ?

Step on the Slug, and save USA.
It ate my foot ? – did not know how big it was ?

Benton Johnson   July 1st, 2008 9:14 am ET

You might have considered the cost per mile to drive as a tool for figuring out if you are making a deal you can live with when you drive. When it only cost 10 cents per mile to drive, going 100 miles was only 10$; however at
50 cents a mile that same trip is 50$. A really big difference based on price for fuel and MPG together.

Dark Coffee   July 1st, 2008 3:39 pm ET

@Joe from florida:

You have to be kidding: 45 MPH speed limits? The police will love you for that, but it won't save much gas and would probably cost lives. Scooters? Ever try to strap a baby to a scooter? Ever try to ride one in Detroit or Chicago during the winter? These things might be fine for a Del Webb retirement community in the sunbelt, but America is a big country.

I agree with you 100% on promoting the high MPG vehicles already being produced, but I have a feeling that $4 gas is promoting them all on its own.

Bill from Nebraska   July 1st, 2008 4:33 pm ET

CNN .COM July 1, 2009

Car sales tumble
Auto sales plunged in June, according to reports from major automakers showing Americans shunning pickups and SUVs in the face of record gas prices, but not yet buying the more fuel-efficient cars as an alternative. reports some of the weak sales may be due to worry about the economy. But automakers also say it could be due to the short supply of fuel-efficient models.


entrepreneur007   July 2nd, 2008 3:28 pm ET

Either way it is not rocket science. This took a "professor" to figure out? Sounds like educated idiots in my book.

Mark   July 3rd, 2008 2:32 pm ET

This is sort of a useless comparison. Obviously a 15mpg car is more efficient than a 10mpg car but the 50mpg car is the best. I don't think anybody is wondering which one of their cars to replace, they're just going to get the most efficient car and drive that one.

Bud   July 3rd, 2008 4:32 pm ET

The chart should take one more leap: 50 mpg burns 2 gallons per 100 miles. To save an additional gallon, one would need to get 100 mpg!

Franko   July 4th, 2008 11:09 pm ET

You could ration, one Slug per week, second Slug at twice the price. .. ...

Or, go electric, throw away your engine and gas tank
uTube; Electric Car in-wheel motor Siemens eCorner
Mitsubishi Wheel Motor

Power from battery, fuel cell, diesel electric, solar cell, exercise bike

Otherwise, invade Canada, Mexico, Iran, Venezuella, Saudi Arabia . ..

Mike   July 5th, 2008 7:03 am ET

Dangit. I really should have paid more attention to that "math" stuff back in school.

Kaen Miller   July 5th, 2008 7:49 pm ET

I want CNN to know about the air car being brought first to India, runs at 65 mph on 2 liters of compressed air

You can leanr more about it at different spots on the web, One spo is www,youtube,com/watch?v=QmqpGZvOY14.

No one ever takes over the air but the will if it is corn, etc.

S Callahan   July 6th, 2008 1:59 pm ET

lol Franko, Sometims in your poems Ican see your point..and sometimes pretty good ones.....
I remember the days of past when we had our gas rationed through odd /even numbered plates..Perhaps it is time to consider this before the winter gets here...just a thought

Franko   July 7th, 2008 5:15 am ET

"Lithium ion battery 0.54 to 0.72 MJ/Kg
melting ice 0.335 MJ/Kg
compressed air at 20 bar (near compression limit) 0.27 MJ/Kg
NiMH Battery 0.22 MJ/Kg
lead acid battery 0.11 MJ/Kg
Gasoline = 34.8 MJ/L 8.9 kWh/liter (generator set produces 3.7 kWh/liter)"

Ec Aims For 100 Km/Liter Car
"12 kWh Li-ion battery system that should support a plug-in hybrid application in a four-passenger vehicle with an all-electric range of 60 miles"

Pedal your excercise bike to compress the air.
Pedal like a horse, for almost an hour, and save 10 cents, drive 5 miiles ?
Faster than mowing the lawn, ferment, then distill the Methanol.
Gasoline may be king per kilogtram, making you a pauper per dollar.

Donovan   September 14th, 2010 5:01 pm ET

the US Government is aware of this- why do you think the cash for clunkers and similar programs only allowed trade in of the lowest gas milage vehicles?

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