Two Ways of Looking at Fuel Efficiency

This is a graphic treatment of an issue mentioned yesterday -- the somewhat misleading nature of the miles per gallon statistic. If you use MPG as your main metric of fuel efficiency, then a change from a 20 MPG vehicle to a 30 MPG vehicle sounds like a smaller advance than does a switch from a 40 MPG vehicle to a 60 MPG vehicle. But if you assume a constant distance to be driven, the former switch reduces fuel consumption more.

Now, obviously, a 20 MPG reduction is still better than a 10 MPG reduction, all else being equal. But "all else," crucially, isn't equal. You get much more bang for your buck by improving performance at the low end.

Comments

One way to raise the low end is to get people out of the bigger cars. Regulate weight (and performance)!

All else being equal, the lighter car will be more efficient than the heavier car.

Posted by Sean | December 24, 2007 1:40 PM

This also doesn't take into different types of vehicles and how many respective miles they travel. Improve semi trucks efficiency by just a few MPG and there a large reduction in overall fuel consumption, simply based on the # of miles they drive.

Check out this story on kite shipping:
http://www.worldchanging.com/archives/007707.html

Posted by Dustin | December 24, 2007 1:44 PM

"Now, obviously, a 20 MPG reduction is still better than a 10 MPG reduction, all else being equal."

Is Matt contemplating a jump to NRO? American Enterprise Institute? OPEC?

Posted by minderbender | December 24, 2007 1:45 PM

The best way to increase fuel efficency is to raise the gas tax.

It doesn't really what the CAFE standard is. It matters what the cost per mile is.

Many of us have a choice of cars to take on a trip.

I am going to visit grandma and, since the whole family is going, I have my choice of cars.

I can cram everyone into a subcompact or I can take our full size conversion van. If the AVERAGE of the MPG equal the CAFE standard then we didn't pay any fines to meet the CAFE standard.

However, if gas is 10 cents a gallon then I will probably take the van and be more comfortable. If gas is $7 a gallon then I will take the most fuel efficient car.

Am I right? or

Is I nuts?

Posted by neil wilson | December 24, 2007 1:56 PM

My car gets crappy fuel mileage because it's old. I don't have enough money to get a newer car.

I guess I have no choice (until I move to a city with better public transportation as well as make more money) but to either guzzle gas or take public transport and then have to wait for an hour for the next bus in case I miss the bus?

But give me money for a newer car (or find me a job in NYC), and I'll personally be making a big impact in terms of fuel usage! ;)

Posted by DAS | December 24, 2007 1:57 PM

I guess I have to ask what the relevance of these posts is, other than to be pedantic. Sure, your mathematics is correct Matt. But what does that mean for our public policy?

Posted by freddiemac | December 24, 2007 1:59 PM

freddiemac: He's saying that past a point, and not even past an especially high point, it's not worth advocating for greater fuel efficiency if people will continue to drive the same distances. If we care about environment/energy management, we need to argue for different modes of population distribution and transportation, rather than sticking to the failed suburbia model that got us into this mess in the first place. We need to induce people, by and large, to live in such a way as to be able to give up their cars altogether.

That's what he should be saying, anyway.

Posted by G C | December 24, 2007 2:04 PM

I guess I have to ask what the relevance of these posts is, other than to be pedantic. Sure, your mathematics is correct Matt. But what does that mean for our public policy?

I too am interested in what Matthew's point is, policy-wise. (Not saying that Matthew has to have a point, just that it would seem like he is leading somewhere.)

I thought I'd note, for example, that the way our CAFE standards are calculated already takes Matthew's point into account. CAFE standards are determined not through a normal "average" of the fuel economies of the components of a manufacturer's fleet, but rather through a harmonic mean. Therefore, when Matthew points out that raising a vehicle's fuel economy from 15MPG to 18MPG improves fuel economy more than increasing from 50MPG to 100MPG, it is likewise the case that a manufacturer increasing a vehicle's fuel economy from 15MPG to 18MPG improves the manufacturer's CAFE more than increasing a different vehicle's fuel economy from 50MPG to 100MPG, all else being equal.

Posted by Al | December 24, 2007 2:05 PM

He explicitly states his policy point: "You get much more bang for your buck by improving performance at the low end." In other words, pushing to increase basic minimum standards >>> making a Super Prius that can get a million trillion miles per gallon.

Posted by Dr. Zeuss | December 24, 2007 2:16 PM

I know I was being a bit obtuse, but Matt's point could be several things here.

hyper fuel efficient hybrids aren't worth the increased cost.

government policy should be focused on limiting gas guzzlers instead of ethanol.

the proposed increases in CAFE won't be effective because it will treat an efficiency of a civic to hybrid civic's mpg of, say 8 mpg as eqivalent to increasing an SUV's efficiency from 18 mpg to 26 mpg, even though we see from the graph that raising the bottom end is better than the top.

Take your pick. I am at a bit of a loss.

Posted by freddiemac | December 24, 2007 2:18 PM

And if you increase from 10 MPG to 12.5 MPG, you'll reduce consumption more than an increase from 50 MPG to 1,000 MPGs.

Posted by Ronnie P. | December 24, 2007 2:22 PM

I think a point of the post could be that if you are numerate and accurately understand the benefits of raising fuel economy across the spectrum, you'll be in a better position to evaluate and propose public policy.

That is enough of a point to justify the post.

Posted by Sean | December 24, 2007 2:22 PM

Come on Matt,
One post about the nature of 1/x was bad enough. But two? I realize its a slow news cycle, but seriously, this is all you could find to blog about?

Posted by michesmith | December 24, 2007 2:29 PM

In Germany and other places in Europe, fuel consumption is commonly given as liters per 100 kilometers, rather than the metric equivalent of miles per gallon. So the misperception Matt describes doesn't arise, interestingly enough. Matt's comparison works out to 11.8 down to 7.8 (for the 20 to 30mpg shift) or 5.9 down to 3.9 (for the 40 to 60mpg shift). It's pretty obvious, in this representation, which is better.

Posted by RSA | December 24, 2007 2:32 PM

Dr. Zeuss, freddiemac:

As I posted above, the way CAFE is calculated already makes it more effective for car manufacturers to increase at the bottom end than the top. Because CAFE uses a harmonic mean calculation rather than simple average, a manufacturer that increases a 15MPG vehicle to 18MPG gets more "credit" toward the CAFE standards than increasing its 50MPG vehicle to 100MPG, all else equal.

Posted by Al | December 24, 2007 2:36 PM

Aaahh... the banality of metrics-quibbling...

(a) Let's christen (!) this Yglesia's Theorem: Different Metrics Measure Different Things. WOW!

(b) Since we've thrown away the all-else-being-equal (AEBE) presumption, then you're merely given us a humungous non-sequitor. Did you WEIGHT each kind of "efficiency change" by the estimated number of vehicles expected to participate in each kind of change? Etc.

Or - lawl - did you want to throw away AEBE *just* long enough to talk about *your* alternative, but then immediately *reinstate it* so as to exclude all of the other possible non-equal-things? Apparently, by your "But if you assume a constant distance to be driven..." it's the latter.

roflmao.

Posted by sherifffruitfly | December 24, 2007 2:37 PM

I should add that the European system makes much more sense to me. You're only rarely in a situation in which you care how far you can go on a fixed amount of gas (e.g., if you're in a desolate stretch between towns with your fuel gauge near empty). What you really want to know if much gas it takes to go a fixed distance (e.g., your commuting distance).

Posted by RSA | December 24, 2007 2:38 PM

Al,

So what exactly is Yglesias saying with these two posts? I mean it is interesting food for thought, but to warrant two posts about it without a direct correlary to government policy is very unMattlike.

Posted by freddiemac | December 24, 2007 2:40 PM

In the European Union fuel consumption is provided in litres per 100 kilometers which provides a direct measure of relative fuel efficiencies.

Volkswagen UK, for example, provides the following fuel consumptions for one of its Golf models:

Urban 29.7mpg - 9.5l/100km
Extra-urban 49.6mpg - 5.7l/100km
Combined 39.8mpg - 7.1l/100km

It is probably expecting too much to have the US start using litres per 100 kilometres but gallons per thousand miles would work. Stores seem to be able to provide cost per ounce for breakfast cereal so why can't car companies do the same for fuel efficiency. Indeed, given the mathematical situation expressed in the original post this might even be advantageous for the US companies still producing large gas-guzzling vehicles.

Posted by ndm | December 24, 2007 3:24 PM

Yes you get more bang for the buck at the low end. And the big auto makers kept exceptions for many of those vehicles. But the reason MattY did a second post is because he REALLY LIKES GRAPHS.

Posted by cheflovesbeer | December 24, 2007 3:40 PM

Re: One way to raise the low end is to get people out of the bigger cars.

Better still: get people out of older cars. Problem there is, how to get poor people driving 15 year old junkers to buy new cars. They can't afford it. Anyone think we could get a car trade-in program where you could turn in any functional old car and get a voucher for a fuel efficient new car? That's what would be needed, but I can easily see the Welfare's Queen's iconic Cadillac being replaced by a Prius in the rhetoric of the Right.

Re: The best way to increase fuel efficency is to raise the gas tax.

See above. That will not help, because altogether too many people cannot afford to buy newer, fuel efficient cars. People who advocate that foolishness are as out of touch with reality as the rightwingers who think tax cuts and HSAs will solve the healthcare mess. Start thinking about real people, not just the upper class!

Posted by JonF | December 24, 2007 4:49 PM

the somewhat misleading nature of the miles per gallon statistic.

Cars are rated using the MPG standard because, on a day to day basis, most of us are asking questions like "If I put 5 gallons in my car, can I get to grandma's house?"

10 MPG x 5 gallons tells me I can travel 50 miles.
20 MPG x 5 gallons tells me I can travel 100 miles.

People setting policy for fuel efficiency standards should be intelligent enough to do the gallons per mile conversion.

Posted by Jinchi | December 24, 2007 7:13 PM

If we care about environment/energy management, we need to argue for different modes of population distribution and transportation, rather than sticking to the failed suburbia model that got us into this mess in the first place. We need to induce people, by and large, to live in such a way as to be able to give up their cars altogether.
That's what he should be saying, anyway.
Posted by G C

Ignored by almost all environmentalists, and deliberately, is America's exploding population growth from mass immigration as the main driving factor of rising energy consumption.

In the early 50s, with 160 million Americans, we were the world's largest petroleum exporters. By 1973, with 225 million people, we imported 30% of our oil needs. By 2006, with 300 million people, we relied on other nations for 70% of our oil, with immigration of just 5 years cancelling out 35 years of conservation and energy efficiency gains since the 1973 Oil crisis.

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Freddiemac - government policy should be focused on limiting gas guzzlers instead of ethanol

Half right. Ethanol from grain is causing diversion of grain from food production to wealthy environmentalists touting their "green renewable fueled vehicles showing they have "no carbon imprint on the planet and more people should strive to be as morally superior as they."

Meanwhile, milk and egg prices have doubled, bread costs 30% more - and the Ethanol lobby screws the poorer Americans and the ones on fixed income. Then they screw them some more, because all the grain diverted into ethanol also gets a heavy taxpayer subsidy from Fed taxpayers and state gas taxes.

Half wrong because "gas guzzler" is just one variable in consumption, others being miles traveled and number of vehicles in use. So 2 million more illegal aliens completely wipes out any CAFE gains in conservation. And retired Ma&Pa Buttercrumb with their "obscene" 7mph RV home they drive 500 miles once a year consume far, far less gas than Ms. Politically Correct San Francisco Power Attorney with her "socially correct Prius" she drives in a 120 mile commute to and from San Fran each day....

Posted by Chris Ford | December 24, 2007 7:57 PM

You're way to impressed that you managed to fully comprehend a jr. high math problem.

Posted by DJ | December 24, 2007 8:52 PM

My car gets crappy fuel mileage because it's old. I don't have enough money to get a newer car.

How much do you actually pay for gas each month? If you have a long daily commute, your car payments might be less then your gas bill on a small used Toyota with great Mileage

On the other hand, if you're not paying much for gas each month, you're not putting out all that much CO2

Posted by Chad Okere | December 25, 2007 12:37 AM

On the inverse-tufte meter this graph rates a solid 8/10, with the remaining two points being spared because the graph is actually not inaccurate. Wow: gallons per mile is the inverse of miles per gallon. In other news, 2+2=4.

But the two areas are in fact identical. ln(30)-ln(20) = ln(60)-ln(40), so what the fock is the point? MY, please contact me before posting another lame plot.

Posted by Matt B | December 25, 2007 2:54 AM

Pretty clearly, if you increase fuel efficiency by 50%, and change nothing else, you will use 1/3 less fuel. What's the controversial insight here?

Posted by Fingal | December 25, 2007 5:02 AM

Still not labeling both axis, but much better than some previous charts.

Posted by cw | December 25, 2007 10:53 AM

The area has nothing to do with anything. It's the vertical drop which we are interested in. Assuming a fixed distance traveled, the position on the vertical axis linearly correlates to gallons consumed.

Posted by greenish | December 25, 2007 8:33 PM