Gallons to Go
- Posted by: GOOD
- on April 28, 2009 at 9:00 am
Knowing your car’s miles per gallon isn’t going to get you very far.
Last summer, Richard Larrick and Jack Soll, professors at Duke’s Fuqua School of Business, published a paper arguing that Americans did not understand how fuel efficiency works. They found that most people assumed that improving a car’s miles per gallon from 25 mpg to 50 mpg would save more gas over 10,000 miles than an improvement from 10 mpg to 20 mpg. But when you do the math, the latter saves more than twice as much (see chart). “Miles per gallon,” they argued, was misleading and did not help drivers understand how much gas they were using. The proposed solution: gallons per mile.
GOOD: So, miles per gallon doesn’t actually measure how much gas you use?
RICHARD LARRICK: The paper was cowritten with a colleague of mine and we actually live near each other and carpool in his Camry hybrid, which has an mpg readout on it. One day, we were watching it and seeing the good and bad mileage. We were thinking about whether you could average together the miles-per-gallon readout to get total miles per gallon, and we realized that the math of miles per gallon gets tricky and it can be really misleading.
Imagine that you are driving uphill for 100 miles and you’re getting 10 miles per gallon, and then you just turned around and drove down the same hill for 100 miles and you got 100 miles per gallon on the way downhill. And the question is: What is your average miles per gallon over that distance?
It feels like it should be about 50, but it turns out it’s 20.
The amount of gas you’re using to go 100 miles when you’re getting 10 mpg is 10 gallons. And when you’re getting 100 mpg as you’re driving 100 miles, you use just one gallon. So you’re using a total of 11 gallons to go 200 miles, and that gets you a little bit under 20 miles per gallon.
G: Do you have any idea how we ended up with this measurement of fuel economy that doesn’t really tell us how much gas we’re using?
RL: [My colleague and I] speculate that when we first had cars, and gas stations were few and far between, maybe it actually mattered that you knew exactly how far you could go on a tank of gas before needing to be able to refill it.
G: Are you seeing more and more people considering using gallons per mile?
RL: A little bit. One of the things we’ve discovered in the process of publishing this and having it be publicized quite a bit over the summer was that conversations like this had gone on at Consumer Reports and car magazines in the past. The engineers know that there’s this problem with miles per gallon. But everyone assumes that because we’re so used to mpg—which we are—that people are not going to be open to changing anything.
I’m kind of frustrated because I’ve tried to reach out to the EPA several times. The one thing they do [on fueleconomy.gov] is gallons per 25 miles. So that is there. And that’s been there since before we did our research. But my problem with that is that 25 miles is too small a distance to actually see the difference in cars. So it’s always .9, 1.2, 1.1—to me, all those numbers of gallons look the same.
We actually prefer 10,000 miles. The key thing about 10,000 miles is that is the distance that many people drive in a year. In fact, they often drive more. It really gives you a sense of, Okay, a year’s worth of driving is going to use 400 gallons, or 700 gallons.
G: What about car companies? Any sense they’ll start using different numbers?
RL: People are always curious—who does this benefit? I’m not really sure if Toyota or Detroit is favored more by this. But I think you can make the argument that it’s Detroit, which was putting hybrids on SUVs and being ridiculed for it. Well, our analysis indicates that’s exactly right. Because to get a car from 14 mpg to 20 mpg is just a huge, huge improvement in reducing gas consumption.
I do know that, in 2004, Honda and Toyota called for supplementing miles per gallon with gallons per 100 miles. And I only discovered this after we published the paper, so it wasn’t something we were able to even cite because we didn’t even know about it at the time. They heard people complaining that the Prius wasn’t getting 50 mpg and that it was getting 42 mpg instead, and people were so frustrated to lose the eight miles per gallon, but once you flip the numbers over you realize you’re talking about a few gallons per hundred miles.
G: So, what’s the next step? How can we use this new knowledge?
RL: This helps us understand that pulling cars out of the teens [in terms of miles per gallon] is so much more valuable than pushing an efficient car even higher. That only becomes clear when you start thinking about gallons per mile. That tiny increase from 10 mpg to 11 mpg saves essentially the same one gallon of gas every 100 miles as does increasing 33 mpg to 50 mpg.
In no way do we advocate that people should stop at 11 mpg, but it at least focuses your attention on getting all those cars in the teens up into the twenties where literally hundreds of gallons of gas will be saved for every 10,000 miles of driving.
DISCUSSION: 14 Comments
I don’t think this was written very well. I’m still kinda confused as to why gallons per mile is better. Why isn’t gpm subject to the same fuzzy math as mpg?
This idea didn’t make a splash when it showed up on blogs last summer either. A car that gets 25 MPG willstill use twice as much fuel as one that gets 50 mpg, just like it alwasy has. That’s a fact. We all understand that. As consumers who want to purchase cars that use less gas that is all you need to know. Nothing has changed.The world needs to rapidly move toward cars that get twice the gas mileage of today’s average. The gallons per mile concept promotes SUVs that get slightly better miles per gallon than other gas hog SUVs.If you are confused just remember this: a car with a higher mpg (like 50) will use less gas than one with lower mpg (like 25) just like it always has. http://www.biodiversivist.com
How about changing fro 10 mpg to 50 mpg – isn’t that better than all…? This is a pretty academic concept and sort of besides the point.
Would be interested in your thoughts on comparing two alternatives to the current gas tax approach (flat rate per gallon): vehicle mileage tax (VMT) versus efficient vehicle assessment (EVA). The former tracks and taxes according to mileage, the latter assesses the per gallon tax according to vehicle make/model/year/weight, etc. (VIN data). I believe EVA does much more to promote a switch to fuel efficient (or non-gas) vehicles and doesn’t rile privacy advocates and rural drivers.
Here in Canada there is a mix of metric and imperial measurements. As a result we get fuel efficiency quoted in both miles per gallon (MPG) and litres per 100km (L/100km). While the L/100km figure is easier to work with since it is linearly related to the amount of fuel a particular trip or commute will take, MPG seems to be the more typically used measure, probably due to the fact that the numbers are more easily comparable as the article mentions.
The idea has practical application. People do think about efficiency improvements on their current vehicles, but they think about it incorrectly: They calculate gas savings by subtracting one car’s MPG from another. MPG can’t be subtracted. GPM can be subtracted to know gas savings.GPM also illustrates more clearly than MPG the urgency of getting people out of inefficient cars. Here is how different levels of MPG roughly translate to gallons of gas consumed over 10,000 miles (GPM). Each improvement saves 100 gallons of gas, or one ton of CO2:10.0 MPG = 1000 GPM11.0 MPG = 900 GPM12.5 MPG = 800 GPM14.0 MPG = 700 GPM16.5 MPG = 600 GPM20.0 MPG = 500 GPM25.0 MPG = 400 GPM33.0 MPG = 300 GPM50.0 MPG = 200 GPMhttp://www.mpgillusion.com/2009/02/overveiw-of-gpm.html
@anon: “The world needs to rapidly move toward cars that get twice the gas mileage of today’s average.”No, for real improvement, the world needs to move away from the idea of personal transportation vehicles for every human being.
This is one of the most confusing articles I’ve read in a longtime! On top of that it really doesn’t prove anything other than you can look at numbers in several different ways! No matter how you look at it a higher MPG number is better. If we were to change the concept and look at gallons per mile a smaller number would be better and every one would be confused for a long period of time.
It’s really not as hard as some people here think. I moved from USA to NZ, and changing my thinking from mpg to litres/100km wasn’t as difficult as I thought it would be. It took me about a month and I got the idea. 5L / 100 km is great, 10L / 100 km is not. Most efficient small cars are 6-7 L / 100 km. Really easy.
one more thing- what is really confusing is why the USA is the ONLY country refusing to get with the program and go metric. Metric is multiples of 10. Way, way easier to understand, if you give it a few minutes. Or are forced to give it a few minutes.
What is wrong with people these days? Is noone ever pleased? The guy spend probably months thinking this up and crunching numbers etc. so how can you expect to understand it as well as him by reading a page? go put in some effort and figure out while mpg is different than gpm. Also, going from 10-50mpg is an astronomical jump, and, like you said, rather beside the point. The man is trying to provide clarity to the masses, something that is much needed, who does not claim to be a combustion engine engineer or designer.
The top graphic tells it all, take the time to see what it’s saying. The improvement from 10 to 20 mpg saves more than twice as much fuel (5 gallons) than the improvement of 25 to 50 mpg (2 gallons).Personally, I learned how to spend 20% less on gasoline. I simply drive 20% fewer miles. Doesn’t cost a cent, and I didn’t have to upgrade from a Ford Escort to a Prius.
Agree with the last few posts – this is a well written and logical
article, pushing a very valid cause. The essential problem with MPG is
that it is a linear scale of numbers representing something non-linear.
This means that one cannot compare different improvements in MPG, going
from 25 to 26 MPG is a bigger change in fuel efficiency than going from
26 to 27 MPG – very confusing. If one were to graph MPG (x axis)
against fuel efficiency (y axis), the line would start from 0,0 (bad
MPG, bad efficiency) and go straight up (great increases in efficiency
for each additional mile per gallon) before curving off to the right
and eventually flattening off (very small increases in efficiency for
each additional mile per gallon). GPM (or better: Litres per Kilometre
- L/km) is a linear scale, representing something linear – quite
sensible. This allows direct comparability for different changes in GPM
or L/100km (going from 8 to 7 L/100km is the same change in fuel
efficiency as going from 7 to 6 L/100km). Thanks to GOOD for covering
this crucial issue. Cheers, B
Interestingly the BBC has just used a deceptive MPG diagram to show Obama’s new efficiency standards. http://news.bbc.co.uk/2/hi/americas/8056908.stm the diagram makes it look like the increase in effiiency for cars is much more than for vans, when in reality they are very similar increases. Vans have a 6,9MPG increase, cars have a 11.5MPG increase. but if you look at the much more meaningful “Gallons per 100 Miles” (shall we say GP100M), the van gets a 1.00 GP100M increase and the car a 1.07 GP100M increase – pretty much the same increase in efficency. a good example of the issues of MPG. -B