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Page added on November 3, 2014
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 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.
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.
19 Comments on "It is Time to Stop Measuring Fuel Efficiency in Miles Per Gallon"
GregT on Mon, 3rd Nov 2014 10:32 am
This article completely misses the point. Fuel efficiency makes no difference as CO2 is accumulative in the environment. More efficient vehicles just take longer to do immense damage to the planet. The end result is the same.
ghung on Mon, 3rd Nov 2014 11:09 am
I doubt this would make much difference in peoples’ perceptions. They may as well rate them as to pounds of CO2 emissions per hundred miles(kg per 100 kilometers?) Likely just as meaningless.
I dropped in at a friend’s Halloween party for a few minutes and listened to some guy explain that CO2 wasn’t an issue with cars because of catalytic converters “which do nothing but hurt gas mileage anyway” (?!) He became quite angry when I pointed out the fallacies in his argument. On my way out, my friend’s wife suggested I keep my mouth shut about things I know nothing about. Too much ethanol in their tanks, it seems.
I don’t think changing how we measure fuel efficiency for cars will make a bit of difference to most folks.
Don on Mon, 3rd Nov 2014 11:30 am
How about before we go switching to the metric system we actually fix it first. When 1 ml is precisely equivalent to 1 cc then maybe we can think about making such a huge change.
herrmeier on Mon, 3rd Nov 2014 11:38 am
I wish every nation had their own system of measure. Would make the earth more interesting.
No matter where you go, it’s always meter and liter. I’d rather have it interesting instead of efficient.
bobinget on Mon, 3rd Nov 2014 11:38 am
Generate one’s own power and a person saves 100%
with an electric vehicle.
You say: Hey, what about the expense of that wind machine or solar panels? How about upkeep? Deprecation?
OKAY, but today a homeowner can simply add solar
to the mortgage, interest deductible, solar credits a bottom line tax credit in many states.
shortonoil on Mon, 3rd Nov 2014 11:40 am
Another example of were an obscure metric is used which distorts perspective is barrels of oil versus BTU. Barrels of oil are no more similar than trees in a forest. Some trees can make huge beams, most will only make toothpicks. Until oil is understood for its energy producing capabilities (BTU), we will continue to shuffle around inconsistent numbers, and never comprehend the meaning, or consequences of depletion.
http://www.thehillsgroup.org/
bobinget on Mon, 3rd Nov 2014 11:41 am
In my state, (Oregon) lawmakers are going to replace the gasoline tax (revenue dropping like rock slide) with a milage tax that will nab those freeloading propane,
and electric vehicles.
.5mt on Mon, 3rd Nov 2014 11:50 am
“The are two major measurement systems currently, one was used to go to the moon, the other is the metric system.”
/who was that, and was the Saturn 5 based on the English system?
Still funny. Sounds like PJ O’Rourke.
ghung on Mon, 3rd Nov 2014 11:50 am
Yeah, Bob, our credit for our new water heating system is 30% federal + 35% state. With me doing the work and previously installed tank, our out-of-pocket expense will be about $700, pretty much what a local plumber will charge for installing a new conventional water heater, more for a tankless.
We did the PV system on our own, over time. The modular nature of PV allows folks to add on over time. We now have enough surplus, on average, to charge an EV (our next big purchase). Upkeep has been a non-issue.
baptised on Mon, 3rd Nov 2014 2:05 pm
Maybe I do live in a good location. If you have your new water heater($150) and take away old, their are plenty plumbers that would hook it up for under $50.Where is it $700?
Perk Earl on Mon, 3rd Nov 2014 2:25 pm
http://www.bloomberg.com/energy/
just interjecting latest oil price:
WTI -1.96 to 78.58
Brent -1.36 to 84.50
Wow, I thought this stuff had stopped going down.
ghung on Mon, 3rd Nov 2014 3:07 pm
Yeah, baptised, your area must be baptised or something. Here in retirement land, most plumbers won’t even show up for under $100. They’ll charge $450 for a descent water heater and another $250 for the install Includes connection to the temp/press valve, new stand and drip pan.
A local plumber (with the big Christian fish on his logo) stuck my old neighbor $240 for replacing a p-trap on the kitchen sink. Told her it cost more because he had to replace the metal pipe with metal pipe. I think they go to the same church.
baptised on Mon, 3rd Nov 2014 4:42 pm
Ghung the way you have it phrased,I thought you meant just “for installing a new conventional water heater”. Now I understand, still a little high for TN.
Amir on Mon, 3rd Nov 2014 7:17 pm
Shouldn’t we be labeling cars with a CO2 per mile rating? That’s the real problem.
adamx on Mon, 3rd Nov 2014 9:51 pm
This article highlights one key issue that is frequently missing in discussions of the sort, that MPG is not the best measure of fuel consumption… But also that saving gas on the fuel hogs is much, MUCH more important overall than pushing it up a bit higher for already efficient vehicles (though every bit counts).
When we talk about passenger vehicles, we should be coming down hard on the SUVs and trucks that are primarily used as personal transport. Instead, CAFE promotes them by it’s rule system which gives them much laxer standards than cars.
On top of that, we have the issue of commercial vehicles. The emissions and consumption of all types of commerical vehicles should be policed heavily, and regulations should promote the development of significantly more efficient commerical vehicles (as CAFE did with cars).
The other end of the scale, extremely small motors, have primarily problems of pollution (including noise pollution) that badly need to be dealt with. The best way is to phase them out for electrics. This is becoming more doable every year, as electrics become more and more powerful.
Overall, though, it’s may just be rearranging the deck chairs on the Titanic. What we really need is to get off of this stuff… Still, I wish the system was logical.
Northwest Resident on Mon, 3rd Nov 2014 10:45 pm
adamx — You raise a question that I have pondered at times in the past. WHY do the automobile manufacturers and their dealers keep producing SUVs and gas hogs of all types when clearly we are running out of oil and the one sure thing that could be done to dramatically lower fuel consumption is to get all of the gas hogs off the road?
I can’t claim to know the real answer, but I think I have a pretty good guess.
First, because all of the auto manufacturers already have their assembly lines built out. All of their parts suppliers are already tooled up. Their advertisements have been designed to impact certain target audiences well in advance. Public Relations campaigns have been launched to lay the ground for the next wave of shiny gas guzzlers. That represents billions and maybe even trillions in investment. They can’t just throw it away and start from scratch.
And why, given the fact that they should have known many years ago that lack of oil was going to someday become a problem, did they tool up to sell gas hogs? Because, that’s what the consumers demanded!! And because just like most other competitive industries in this world, the CEOs had to worry about short term consequences, long term consequences be damned. Stupidity and greed and shortsightedness ruled the day and lead us directly to where we are today.
Then there’s the question, seeing now how badly they screwed things up, why doesn’t the government just mandate those gas hogs off the road and force everybody into smaller, more economical models?
Several reasons:
1) Many businesses depend on those gas hogs to get their work done — their business model is built around burning a LOT of gas
2) Too many cowboys, rednecks, good old boys, suburban dudes and dudettes and others would scream and take revenge at the ballot box if they were forced to part with their beloved gas guzzlers. Sure, if all politicians could get on the same page and explain the dilemma and the need, that might help. But there are many politicians who would use it as an issue to sway voters, seeking of course to get more power, more money — the same exact behaviors that got us to where we are today.
Seeing all this and knowing all this, I feel certain that the US Government has come to a solid conclusion of “screw it, we’re going down anyway, this problem will resolve itself in short order.”
If the system were logical, we wouldn’t be in this mess, not even close. But it is the exact opposite of logical, which is representation of the consolidated mass of humanity — totally, completely illogical. And so, we’re going to have to pay the price for all that stupidity and greed.
Davy on Tue, 4th Nov 2014 6:43 am
NR, I hear you man but look at it this way. Many of these big trucks and SUV’s have great salvage value as all terrain people movers and small freighters. They are built tough to last. They have poor applications now as SOV’s for sure.
Tell me how long all these fancy high mileage cars are going to make it with potholes and sprouts growing out of the pavement. Why is ISIS running around in Ford and Chevy trucks? In a collapsed world the toughest equipment will have a second life. American SUV’s have the potential for utilization in a collapsing world.
Northwest Resident on Tue, 4th Nov 2014 9:54 am
Davy — You just inspired me to get a copy of “Road Warrior” and watch it again! And by the way, thanks for bursting my bubble. I was starting to dream of post-collapse (assuming I make it) as a return to an idyllic life of agricultural pursuits, tight-knit bonding local community and nature healing herself. Then, bursting into my dream, here comes a rumbling F350 Diesel belching smoke, rattling and bumping through the potholes and sounding like a race car with no muffler. Time to wake up! 🙂
GregT on Tue, 4th Nov 2014 10:31 am
“Why is ISIS running around in Ford and Chevy trucks?”
Good for business. We wouldn’t want them driving Ladas, or Dartzes now would we? Besides, there appears to be a shortage of whale penises. 🙂
http://jalopnik.com/5380680/15m-russian-suv-features-diamonds-whale-penis-leather