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It is Time to Stop Measuring Fuel Efficiency in Miles Per Gallon

Fuels and Efficiency Measurement Standards

Due to the absurdities of history, America uses what is essentially a mediaeval unit system that appears completely untouched by the scientific revolution. Corn yields are measured in bushels; slabs of meat are measured in pounds; distances are measured in miles, feet and inches. Why such units have not been recognised as a menace to public understanding – and hence abandoned – is a mystery that perhaps will one day be resolved by historians. For now we can only marvel that the world’s most technologically sophisticated country is composed of citizens forced to figure out how many feet are in a mile, a task that was made redundant with the invention of the obviously superior metric system two centuries ago.

Bad or inappropriate units unfortunately have consequences. Humans have evolved to be comfortable with little more than small numbers. Tribes on the African Savanna did not need to have a feel for what a trillion might be. The fingers on your hand may have been enough hundreds of thousands of years ago. Today we must expand our numerical comfort zone by many orders of magnitude.

My favourite illustration of this comes from the great physicist Richard Feynman. There are 1011 stars in the galaxy. One hundred billion; this, then, is an astronomical number. Yet, as Feynman pointed out it was less than the national debt at the time. He said this long before he died in 1988. I don’t need to point out that the national debt today is more than astronomical. Evolution has granted us weaker numerical skills than we now need, and we make this problem worse by continuing to use units which lead our intuitions down garden paths.

The Miles Per Gallon Illusion

A real world example of the consequences of bad units is the MPG Illusion; a term first coined, I believe, in an opinion piece in the journal Science in 2008. Miles per gallon is a quaint and ostensibly harmless way to measure the fuel efficiency of a car. In reality it just results in consumers and policy makers making bad decisions.

American cars today are as heavy as they were forty years ago. Meanwhile, the average fuel efficiency of the American car fleet is so low compared with the standards of developed countries that it should rank alongside the preponderance of belief in creationism as a supreme national embarrassment.

Drastically reducing the air pollution and greenhouse gas emissions from America’s cars should be a national priority. It is both essential and relatively easy. After all, if the US Government issued a fiat tomorrow requiring everyone to drive a Honda Civic there is not likely to be an epidemic of depression in response.

However, public understanding of this vital issue is clouded by the archaic way fuel efficiency is measured. This is demonstrated by a simple example.

Imagine that two people are considering buying new cars. One is thinking about a switch from a 20 to a 25 MPG car, the other from a 25 to a 30 MPG car. In both cases the fuel efficiency of the car improves by 5 MPG. Which driver will save the most fuel?

Most people believe that the fuel saved is the same for both drivers.

This is wrong. The first driver actually saves 50% more fuel. To see why, let me re-phrase my simple example.

Two women are considering upgrading their cars. One is thinking about switching from a 5 gallons per 100 miles car to a 4 gallons per 100 miles car. A second woman is thinking of switching from a 4 gallons per 100 miles car to a 3.33 gallons per 100 miles car. Which woman will save the most fuel?

You can clearly see that it is the first woman. She saves 1 gallon per 100 miles, whereas the second saves 0.667 gallons per 100 miles.

This can be demonstrated graphically as well. Let us start with a 20 MPG car and consider how much fuel is saved per 100 miles as we improve the fuel efficiency of the car. The savings per 100 miles for all fuel efficiencies up to 50 MPG are shown below.

mpg

People’s intutions tell them that a 5 MPG improvement will always save the same amount of fuel. Far from it. The fuel saved by switching from a 20 to 28 MPG car is roughly the same as switching from a 28 to a 50 mpg car. One is an 8 MPG improvement, the other is a 22 MPG improvement. Yet, the fuel saved is the same.

The consequences of this are obvious. If you want to reduce the environmental footprint of cars, you should not simply aim to reduce average miles per gallon. The average fuel efficiency of new passenger vehicles on American roads is 24.1 MPG, according to the latest EPA figures. Persuading someone to switch from an average vehicle to a 50 MPG vehicle will have greater impact on average mpg than getting someone to ditch their 15 MPG SUV for an average efficiency vehicle. Yet, getting the SUV driver to switch will reduce fuel consumption more.

So, I will make a two-step proposal. First, we switch from measuring fuel efficiency in miles per gallon to measuring it in gallons per 100 miles. And second, we then switch from gallons per 100 miles to litres per 100 kilometres. But before this we should do something about the absurdity of people driving around in excessively large vehicles that not only needlessly pollute cities, but also do immense damage to the planet.

Robert Wilson's picture

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Discussions

Keith Pickering's picture
Keith Pickering on Nov 3, 2014 3:00 pm GMT

Gallons-per-mile results in fractional numbers, which most people hate or don’t understand.

Instead, we need to use gallons-per-hundred miles.

Robert Wilson's picture
Robert Wilson on Nov 3, 2014 3:36 pm GMT

That was a typo. Now fixed.

Max Kennedy's picture
Max Kennedy on Nov 3, 2014 6:52 pm GMT

L/100km is the norm in Canada and most of the rest of the world.  Wondering when the US will catch up and abandon their archaic confused measurement system.  Though based on the old British system even they have abandoned it!

Bob Meinetz's picture
Bob Meinetz on Nov 3, 2014 7:52 pm GMT

Robert, we have many, many issues which are of supreme national embarrassment. Acknowledged, and grateful to be included in your list of developed countries.

For now, I have to take comfort in the fact our beer is getting better.

Joe Schiewe's picture
Joe Schiewe on Nov 3, 2014 10:15 pm GMT

Would it not be more helpful to use L/20,000 km or tons of CO2/20,000 km (close to typical annual mileage) so that a person could calculate the approximate liters, $$$ and CO2 saved in a year?  

Max Kennedy's picture
Max Kennedy on Nov 4, 2014 12:17 am GMT

And how do you propose to get 50mpg new cars at no cost?  That is simply lala  land.

John Miller's picture
John Miller on Nov 4, 2014 1:47 am GMT

Robert, unfortunately many Consumers do not focus solely on fuel efficiency when making vehicle purchasing decisions.  They tend to first focus on their utility needs, or the required size of the vehicle needed to haul family members (the average number of passengers of course varies), the volume of goods & commodities typically transported (baby seats, strollers, dipper bags, sporting goods, bikes, wheel chains, foodstuffs, suitcases, utility/trailer hauling capacity, etc.), average travel distance used on weekdays & on weekends/holidays, and the availability of alternative transport (buses, trains, planes, etc. or the second family vehicle).  Fuel efficiency or variable operating costs (fuel + maintenance) becomes the next priority in addition to the fully amortized cost of the vehicle purchase.

The priority of fuel costs or fuel efficiency depends on the income level or ability of the Consumer to pay for owning and operating a given vehicle.  Your assumption that average Consumers cannot figure out the overall impacts on operating expenses based on miles per gallon or gallons/liters per 100 miles/kilometers etc. may be true for those with upper income levels or for owners that don’t find the fuel costs significant.  But, most middle/lower income Consumers probably pay more attention to the details such as fuel efficiency when making vehicle purchase decisions.

One factor you have overlooked in the possible confusion relative to vehicle fuel efficiency is the recent U.S. EPA’s CAFE standards which are now based on grams CO2 per mile.  

Bas Gresnigt's picture
Bas Gresnigt on Nov 4, 2014 10:01 am GMT

Easy to implement:

1. Raise the fuel tax to NW-European levels.
E.g. in NL normal car fuel price is €0.66 + taxes of €1.06 = €1.72/liter
That is = $8.50/gallon.
Then people will drive more slowly.

2. Commit for a regular increase of the tax during the next 20years.
E.g. each year with a % that is five times the inflation rate, and announce that yearly increase.

So people will not only drive more slowly, but also buy only very fuel-efficient cars.
Plug-in hybrids and EV’s will become popular.

Spend the increased tax revenues partly to lower general taxes (e.g. VAT), improve public transport, and stimulate renewable.

Clifford Goudey's picture
Clifford Goudey on Nov 6, 2014 9:13 pm GMT

Robert, you wrote, “People’s intutions tell them that a 5 MPG improvement will always save same amount of fuel.”  Not the people I know, but over the range of typical cars in the US fleet it really matters little.  And for cars in the fleets of countries where good mileage is valued, it matters even less.  The incremental saving from 55 mpg to 60 mpg is very small.

However, a 5 mpg improvement in a tractor trailer can represent a doubling of fuel efficiency.  I think most people’s can grasp that, don’t you?

Geoffrey Styles's picture
Geoffrey Styles on Nov 6, 2014 9:51 pm GMT

Really? In my experience, despite paying the taxes you highlight above, typical Europeans drive faster than most Americans, and EVs and PHEVs are more popular here, with $3 gas than there, with $8 gas.  

Geoffrey Styles's picture
Geoffrey Styles on Nov 6, 2014 10:13 pm GMT

Robert,

I’ve been writing about this for years, too, and share your frustration with mpg. The European formulation of volume/distance is clearly more intuitive and informative. However, another factor that makes both mpg and gal/100 mi. problematic is the diversification of transportation energy. The federal government has attempted to bridge this with MPGe (miles per gallon-equivalent) but even that (or its reciprocal) is subject to big assumptions about appropriate energy conversion rates.

When I looked at this issue in 2008 It struck me that what consumers really care most about is neither mpg nor gal/100 mi. but miles per dollar. I still think that’s the winning formulation for the long term, and it would help drive cleaner fuels to become cheaper, too.

martin burkle's picture
martin burkle on Nov 6, 2014 10:27 pm GMT

What would you suggest for electric cars? both the hybred and the all electric

I think that cost per 1000 miles is a better number when buying a car. The larger number of miles makes it easier to think about yeary cost-of-ownership.

Max Kennedy's picture
Max Kennedy on Nov 6, 2014 11:04 pm GMT

It’s too bad “most people” didn’t pay attention to ratio’s in high school math.  Some of that schoolin’ they’ll never use in real life I guess!

Bas Gresnigt's picture
Bas Gresnigt on Nov 6, 2014 11:28 pm GMT

Agree. Especially on German highway’s as those often have no speed limit.

However, I have the strong impression that in the past years even there, the speeds are lower now. 
Gas prices are now much higher than they were (~€1.70/liter now).

Little doubt that with a gas price of €10/liter (=$50/gallon) nearly everybody will look around for more economic cars and drive slower.

Not sure why EV’s are more popular in US.
May be the owners get better (tax) benefits in US, or the Tesla models are a little too fancy (& big) for most Europeans?

A Siegel's picture
A Siegel on Nov 7, 2014 2:23 pm GMT

If we included in gasoline, repairs, insurance, etc … E.g., is it the real cost per 1000 or simply the fuel cost that merits discussion/highlighting?

A Siegel's picture
A Siegel on Nov 7, 2014 3:36 pm GMT

Geoffrey / Robert — also one who has discussed this in past. (See, for example, http://getenergysmartnow.com/2009/06/11/a-note-why-gpm-makes-for-better-...)  

RE mpg illusion, worth linking to/mentioning the site: http://www.mpgillusion.com/  Discussion there is not just re mpg/gpm but where other measures confuse … such as, top of the posts right now, how people don’t understand sunscreen SPF factors.

Re price per mile, as mentioned above there is the serious issue of how do we help people understand total cost of ownership (capital cost, general repairs, insurance, …) versus incremental costs (increased maintenance costs, perhaps increased insurance costs, fuel) versus solely the fuel cost. Most people, in my understanding, simply think the last to the extent that they consider the cost to drive a mile.  Thinking the first helps people understand / think about ‘do we really need two cars in the family or does it make sense to have one and rely more on public transportation then rent a car or use a taxi when there is really is a need for that second one.

A Siegel's picture
A Siegel on Nov 7, 2014 3:36 pm GMT

Geoffrey / Robert — also one who has discussed this in past. (See, for example, http://getenergysmartnow.com/2009/06/11/a-note-why-gpm-makes-for-better-...)  

RE mpg illusion, worth linking to/mentioning the site: http://www.mpgillusion.com/  Discussion there is not just re mpg/gpm but where other measures confuse … such as, top of the posts right now, how people don’t understand sunscreen SPF factors.

Re price per mile, as mentioned above there is the serious issue of how do we help people understand total cost of ownership (capital cost, general repairs, insurance, …) versus incremental costs (increased maintenance costs, perhaps increased insurance costs, fuel) versus solely the fuel cost. Most people, in my understanding, simply think the last to the extent that they consider the cost to drive a mile.  Thinking the first helps people understand / think about ‘do we really need two cars in the family or does it make sense to have one and rely more on public transportation then rent a car or use a taxi when there is really is a need for that second one.

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