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

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 areÂ 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.

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.