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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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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.

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.

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!

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.

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.

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

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