| Why MPG is a Stupid Measurement |
| Written by Hank Green | ||
| Monday, 14 July 2008 | ||
|
So it should come as no surprise to you that measuring vehicle efficiency in miles per gallon is about as effective a city's population density in square miles per person. The reason is simple. What we want to know is how many gallons we'll save. But what we're getting is how many miles we can go. This might seem like a small difference, but displaying miles per gallon in gallons per mile immediately shows some huge flaws in our current system 15 mpg = 660 gallons per 10,000 miles Now, obviously, the most fuel efficient car here is still the 60 mpg car. And that's fantastic. But increasing the mileage of a 15 mpg car to 20 mpg, saves as much gasoline (and carbon) as doubling the mileage of a 30 mpg car to 60 mpg. But to the consumer's eye, the difference from 15 to 20 might not seem all that important. One might even go so far as to say that they would see it as insignificant. If they're going to buy an inefficient car, what does it matter if they get an extra five miles for every one gallon of gas. This, in short, is why the Chevy Tahoe Hybrid won Green Car of the Year this year. And while it may seem like the best way to save gas is to make small cars more and more efficient, it's obviously much more important to work on the big cars first. Not only is a gallons per 10,000 mile system more accurate in terms of efficiency, it's better for the consumer. It shows them exactly how many gallons they'll be burning. And, from there, it's only a quick multiplication to get yourself a big scary number for your gas bills. Of course, if history is any guide, switching the units will be no easy task. But the next time you're at a dealership, maybe you should bust out your calculator and figure out how many gallons per mile you'll be getting.
Comments
(46)
I disagre
written by Clinch , July 14, 2008
...
written by haichen , July 14, 2008
No, what you really want to know is: How much does it cost driving to work or somewhere else. You can calculate it in your Brain if it is gallons / 10000 miles or in Europe liter/100km. Try this with mpg.
More Confusion
written by Techfrog , July 14, 2008
How is that more accurate? at any given moment, we know what a gallon of gas costs. We now know that figure will take us 15, 30, or 40 miles.
10,000 miles takes about eight months to a year for most, so whats the point of that calculation? Fuel prices vary over that period, so to do a long term calculation is meaningless. In your linear way of looking at it, you've actually managed to make it more difficult. 40mpg is easy. Buy a gallon, go 40 miles. End of story.
...
written by EV , July 14, 2008 You can calculate it in your Brain if it is gallons / 10000 miles or in Europe liter/100km. Try this with mpg. Easy. Miles/mpg*($/gallon). As opposed to your way of Miles*(Gallons/Miles)*($/Gallon). I'm not seeing what the big difference is other than a division in place of a multiplication.
Why so great?
written by neilgx , July 14, 2008
If the MPG measurement is so bad, why is the inverse (GPM) so great? Why not use the MIN and MAX amount of fuel used per revolution of the engine as a measurement of efficiency?
Treehugger had an article about this
written by allie , July 14, 2008
Here's a link: http://www.treehugger.com/file...ng-mpg.php
The gist of it is that the relationship of miles to gallons is curvilinear, not linear. So replacing a car that gets 12 mpg with a car that gets 14 mpg gives you greater savings than replacing a car that gets 28 mpg with one that gets 40 mpg. Since it isn't linear, it confuses a lot of people, and it isn't a realistic measurement. Gallons per mile gives you a better, more realistic representation of how much fuel you'll be using on any given trip.
who cares?
written by lennie , July 14, 2008
I think is that most of the big car and light truck makers could easly pump out 2 two 4 times the MPG that thay get now.
RE:More Confusion
written by lo , July 14, 2008
No, it's not more accurate, it's just a MUCH BETTER WAY to represent the same number.
It's not "more confusion", nor is it difficult to understand why this is a better way as it's spelled out right in the post: people do not regard a 15MPG->20MPG increase as anything significant, even though it is an 3 increase, and actually reduces the absolute amount of fuel consumed over a certain distance the same as going from 30MPG to 60MPG. Again, the data is the same, the relations are just being shown different. But this is a SIGNIFICANT IDEA and one in which I was happily surprised to find. I always find it interesting how difficult it is for humans to compare and contrast differing amounts and percentages... something in the brain.
...
written by Ken Roberts , July 14, 2008
Perhaps both numbers could be displayed on automobile stickers. Easy compromise that gives more information to the mathematically inclined and doesn't confuse your grandmother.
...
written by Lloyd Alter , July 14, 2008
the problem is one of comfort and custom. We have had the metric system for 25 years, buy our gas in liters, and measure fuel economy in liters per 100KM, yet the first thing everyone wants to know is, how much is that in miles per gallon, even though we have not driven miles or pumped gallons in years. Habit it tough to beat.
...
written by Clinch , July 14, 2008
The reason the treehugger linked graph is curved, is because you've comparing a unit of measurement to its inverse. If they put the graph as x=GPM, against y=distance traveled on a gallon of fuel (in miles), the graph would also be non-linear.
And you can just as easily use the inverse argument. Comparing two change; A-Changing from 30GP*M to 20GP*M B-Changing from 300GP*M to 200GP*M It may seem that B is a better option, because you save 100 gallons compared to only saving 10 in A. But if you put it in to MPG A- 333MPG to 500MPG B- 33MPG to 50MPG You see that A can do an additional 167 miles per gallon, compared to B, only 17. And in the end, when are you ever going even consider buying, for example, an 20MPG SUV, because the increase in efficiency compared to its previous model is five times that of the efficiency increase of a 70MPG hybrid compaired to its previous model?
I am Agree
written by Daniel , July 14, 2008
I am agree with you!
RE:RE:More Confusion
written by Clinch , July 14, 2008
You may save the same amount of fuel when changing from 15mpg to 20mpg, as you do when changing from 30mpg to 60mpg, but using GPM gives no indication of the overall change in efficiency.
Saving 50 gallons wouldn't be that big of a deal to someone who uses 600GPM, as it's only an increase of efficiency by 8%, but to someone who gets 100GPM, saving the same 50 gallons, would be doubling their efficiency.
Don't forget US Gallons are different to
written by litsl , July 14, 2008
If you need another reason to be concerned about MPG it's also worth remembering that US MPG is different from UK MPG. A US Gallon is 3.785 litres whereas an imperial gallon is 4.55 litres. Not great for making comparisons.
Just a quick heads up from the UK where gas is $9 dollars per US Gallon.
Metric / Imperial debate
written by Fox , July 14, 2008
Never forget that a very expensive NASA Orbiter to Mars crashed because of a imperial/metric measurement confusion !
http://www.cnn.com/TECH/space/9909/30/mars.metric.02/
Speed limits
written by Daniel Rossi , July 14, 2008
Why we still make cars that run above the speed limits. Of course if you give me a car that goes above 100 m/h sooner or later I will push the gas to test it, which is cool and boost my adrenaline but I dont want a car like that even electric. Speed limits should not be above 55 mpg.
...
written by EV , July 14, 2008 Why we still make cars that run above the speed limits A few reasons, really. One is that the peak efficiency of an engine is below it's max power. This is true for any engine that I know of. Two is that, occasionally, you need the power that can take you above 55mph when you are well below 55mph. It takes more power to accelerate from a stop to 20 in a reasonable amount of time than is used to maintain 55mph. Or say, you get into a situation where you need to accelerate quickly, say to avoid the idiot who is about to sideswipe you at an intersection. Three, you also occasionally need to go above 55. Such as when the car next to you on the interstate is trying to move into your front seat. Either that, or you slam on the brake real fast. Both things have happened to me in the past few years. There are real, practical reasons for designing cars this way. Some of them are even safety related.
...
written by L/100km, like the rest of the world , July 14, 2008
L/100km. The entire world can do it, so can you.
a better way
written by Jacob , July 14, 2008
ok someone says that gallons per mile is better, so lets see:
15mpg = 0.07 gpm 20mpg = 0.05 gpm 30mpg = 0.03 gpm 60mpg = 0.02 gpm 120mpg= 0.01 gpm No, not that helpful. What would be a better way of presenting mpg information to the car buyer? How about a graph? Like this one I put up: http://greendreamsinmpg.blogspot.com/
Can the USA catch up?
written by Joe , July 14, 2008
How much energy is wasted in converting US exports to metric and vice versa for US imports? No doubt may tons of C02 are added to the atmosphere as a result of this wasted effort. A great challenge for the USA would be to go metric at last and join the rest of the world, the energy savings could be huge. Is there any support for this in the US?
At least we all agree on something..
written by Kevin , July 14, 2008
US definitely need to follow the crowd sometime.. It's stupid to have different metrics than the rest of the world.
Car Show
written by The Food Monster , July 15, 2008
http://thefoodmonsterblog.blogspot.com
There was a Car Show on the radio in So Cal, maybe it was Car Talk, or the Car Guys, but they took a half hour and explained this as well. Although, this sums it up quite nicely.
The rest ot the world uses litres per 10
written by Australian reader , July 15, 2008
Thanks for this Hank. In Australia we use litres per 100km as the efficiency measure and MPG always seemed to me to be an odd measure. Now I can understand why.
tahoe
written by IamIan , July 15, 2008
It doesn't matter how many people drive non green overly polluting vehicles like tahoes... they are still non green and overly polluting... a HEV Tahoe is better than the non HEV version , and is geenER... but even the HEV Tahoe is overly polluting and non green... that is why allot of people do not like the award going to it... The award gives the confusing message that the HEV Tahoe is a greener vehicle than something like a Prius... that somehow the world would be better off if everyone driving a Prius would trade it in for a Tahoe since it is the greenest vehicle.
I do not believe in lowering the bar like they did with that Tahoe Award... people who trade in a small compact lower polluting vehicle for a large Tahoe should not be given any false ideas that they are driving the greenest vehicle or that they are greener than they were in the smaller car. also, going from 30 MPG to 60 MPG might save the same amount of gas as going from just 15 MPG to 20 MPG... but the 60 MPG should seem more impressive... because the 60 MPG car will use less than 1/4 the fuel of that 15 MPG car... while the 20 MPG car uses 3/4 the fuel of that 15 MPG car... Any increase in MPG is good... and to me how you measure it is the same kind of thing as a favorite color... there is no really wrong way... they all give you the same information... it is just a personal preference.
...
written by Curaitis , July 15, 2008
Good article! I never thought about the disadvantages of the mpg measurement system before. We have the liter/100 kilometers system here and it works quite well. Ironically one problem is the same: It would be more helpful if a larger car needed 10 liters instead of 12 liters on 100 km than a smaller car to need 5 liters instead of 6. But people tend to see the procentual savings which are the same in this case. Otherwise it is of course better to buy the more fuel efficient car
 .
...
written by Craig , July 15, 2008
IamIan hit the nail on the head, while i do agree it is a better way of looking at efficiency, and going from 15-20MPG saves as much gas as going from 30-60MPG. The point is you are still using way more gas if you are driving that 20MPG SUV v the 60MPG vehicle. Giving the Green award to a SUV is ridiculous - for anyone not using it for work purposes or towing, give me a good reason for needing an SUV like that? In every other country in the world, mum's and Dad's manage to ferry the entire family, including the dog, pick up grocery's and carry other stuff without the use of an SUV. So making a wasteful, unnecessary vehicle slightly "greener" does not make it award worthy nor any more ok to drive.
Great article about a new way to look at MPG though, count me in.
Are we looking at the wrong thing?
written by Adam , July 15, 2008
I've heard (maybe at ecogeek, or maybe somewhere else) that it takes far more energy to manufacture a computer than the computer will ever consume in its lifetime. So what's the energy required to make a new car? IF, and only IF, the computer energy-use argument holds for cars, I wonder if, big picture, it'd be worth continuing to drive the 15 mpg thank until its run into the ground rather than buy a new 60 mpg pimp ride? I'd love to see at what point it becomes worthwhile to scrap the used F-150 and pick up a new hippie mobile--but I have no interest in doing the math.
...
written by Jacob , July 15, 2008
Look. As a matter of public policy: if we want to reduce greenhouse gases then we'll get more bang for our buck by getting all cars up to 30mpg. Pushing from 30 to 60 is more expensive and much less of an improvement. At which point it might be more effective to push in other directions with the money (like building efficiency).
...
written by Jacob , July 15, 2008
If it takes far more energy to manufacture a car than it will ever use in its lifetime, then let's see: it would be really expensive. Energy isn't free. You pay for it. So, compare the dollar cost of fueling a vehicle for 20 years and the cost of buying the vehicle. I don't care if its
...
written by Clinch , July 15, 2008
That's why people shouldn't take MPG (or GPM if you've already converted) as a judge of how green a car is, but rather how green it is cradle to grave, but car companies are reluctant to release this information, as many "green" cars (e.g. the prius) would be shown to not be as green as they want people to think they are, and therefore lose the aspect with which they use to sell their cars.
My rebuttal
written by Danno , July 15, 2008
While this is an interesting observation, I played with some numbers to see how this really affected the cost of owning a car and how much it made a difference. The answer will surprise you. (Hint: It doesn't.)
Please check out my blog article at http://allthingscon.blogspot.com/2008/07/ratings-game.html
danno tha'ts fine but
written by Jacob , July 15, 2008
1. you missed the point. the point is that most efficient gains are made on the lower end.
2. It is not always the case that more fuel efficient cars are more expensive than less fuel efficient cars. 3. There are other reasons beyond $$ for why people choose particular cars. For some, efficiency (or environmentalism) is an end in itself. 4. Your conclusion (that we ought to just wait for technology to save our ass) is not warranted by your argument. 5. Your argument assumes that gas won't keep going up. Up or down, which is more likely? 6. Economies of scale can drive down the cost of fuel efficient cars as it is. Your argument doesn't account for the non-technological reasons cars have the fuel efficiencies (and prices) they do.
Plastic or Paper
written by Souptik Gupta , July 15, 2008 Would you rather have a root canal without Novocain or fall down a flight of stairs? Would you rather breathe second hand smoke or get an X-ray without the lead protective vest? Would you like paper or plastic? Please visit http://www.simplewaystohelp.com for all your queries.
ajkasdljfk
written by This email address is being protected from spam bots, you need Javascript enabled to view it , July 15, 2008
The dealerships that I've been to recently have the price of gas for one average year of car ownership pasted to the car's window. I don't know if that's just a California thing, or a national thing, but it has the same basic impact.
...
written by Ken Roberts , July 16, 2008
To me, it's a matter of perspective. Implicit in the two measurement systems is a set of assumptions. The GPM system (or L/100km) has an implicit assumption that you'll be traveling 100km anyway, and want to know how much it will cost you. The MPG measurement assumes that you'll buy X gallons of gas, and then drive around until you run out.
In reality, both are misleading. Our driving habits are heavily impacted by our automobile's fuel consumption, especially if it is significant in relation to our wallet size. A jump of 15 MPG to 20 MPG won't save you as much money as you would expect from the GPM measurement. Since the marginal cost of driving additional miles will be less, you will drive more, and thus consume more gasoline. This is standard economics. From a logical perspective, I prefer the MPG measurement. If I'm low on gasoline, I want to know how much I will need to get me home. If I'm less than 20 miles away and there is no traffic, I may be able to do it with a single gallon. Compare this to a GMP measurement. If your car uses 500 gallons per 10,000 miles, you'd first have to divide 500 by 10,000, giving .05. Then you'd multiple this by the number of miles, giving 20*.05 = 1 gallon. This is simple enough with whole numbers, but I don't want to have to do this kind of math on the highway. Just some thoughts. As I said before, it would be a good idea to present both numbers. There's never such a thing as too much information.
So big efficiency gains at the low end..
written by Ed , July 16, 2008
So a 5 mpg gain at the low end of the fuel economy continuim yields a big gain in gallons? People can probably achieve that (on the highway) just by driving 55 mph instead of 70 mph.
www.drive55.org
...
written by Konrad , July 16, 2008
Thank you Hank!
I always wanted to shout it out to the world: get rid of you crabby measure system!!! liters per 100 km is the easiest way. you can calculate how long you can drive with x liters left in the tank AND you can calculate what it will cost you to drive somewhere as the gas price is always given. now THAT'S what I call a measure system.
@Jacob
written by Danno , July 16, 2008
1. I didn't miss the point. I acknowledged it. My intent wasn't to argue that Ecogeek was wrong, I'm saying that in terms of economics it's something that doesn't have as much an impact as we'd wish. I wasn't trying to pick a fight, I was just trying to make a point about the economics of car buying and how much MPG fit into that.
2. Very true. And that's why the Prius is a hot seller. I was making the observation that all other properties being equal, if two models are differentiated by efficiency and the cost is more for the more efficient vehicle, it will not like provide incentive to buy the more efficient vehicle. 3. Also true. Many people buy green vehicles to "save the planet". Not everyone is so chivalrous - most people are driven by the Almighty Dollar. Which is why it's hard to change people's minds when buying cars if being green costs more. 4. How else do you propose that we create greener vehicles? Magic? If being greener costs more, there's no economic incentive to do so. Technological innovation leads to greener vehicles that are also affordable and price competitive. 5. I clearly stated that I believed that prices were more likely to go up. Anyone who thinks otherwise is fooling themselves. I also stated that the only way for people to have economic incentive to buy greener cars was to allow the price of gas to continue going up. 6. Good point. But once again, I was assuming "all other things being equal", which included economies of scale.
...
written by Logy , July 16, 2008
Why should I care when water freezes or boils? I know that I personally freeze at 0 Fahrenheit and boil at 100. That is a temperature scale that I can relate to.
Great Article
written by Monica , July 16, 2008
You're right, measuring how many gallons per 10,000 miles is a lot more clear than MPG. It's a lot easier to see how much money you will save on gas over the course of a few months or a year. As a student struggling with loans, this is Very Important Information! :-)
Not that it really matters, since I don't have a car...but still. I agree with the article.
...
written by Ray Peacock , July 17, 2008
Units aren't stupid, regardless of your rabble-rousing rhetoric. Do something useful instead of wasting my time to follow a Google News lead to this frivolous nonsense that you wasted your time writing and expounding.
So much trivial crap, so many real things to be done, like improving health care, our government, our education systems, getting people to vote, preventive health care, affordable health insurance, improving thermal energy efficiency throughout the country (Did you hear Clinton's speech to the National Governors' Convention and the challenge he set before them on FREE energy savings in State Buildings and how it can work- There's something you might want to get your teeth into).
Operating $'s spent
written by Bill Lorch , July 19, 2008
OK here is the real deal. Fill the tank,record the miles! drive where you are going to . Fill the tank record the mileage!! How many mpg did you get? now divide this in2 cost per gallon!! Walla you have much it costs u in fuel to go 1 mile. How many miles do you drive a yr.?? You can now accurately get the cost of fuel a yr. Now please don't forget the amt. of engine oil u used, Did you buy tires? All this adds to the operating cost . Don't forget the insurance. A car is an extremely expensive proposition...
displays in cars
written by Dan Leithauser , July 20, 2008
I think the most useful display on the dash of a car can be the instantaneous MPG measurement. I have an 2007 Chevy Impala. Having this display on helps you understand and learn how to optimize fuel usage. The most revealing? On a level portion of highway.. at 75 mph the car is getting 23 MPG, at 55 mph--28 MPG, at 45 mph--37 MPG! (AC running). Overall fuel economy would improve simply by driving more steadily and slower, but of course, that is not how people *like* to drive is it?
Standardization of terms.
written by EvanB , July 20, 2008
GPM tends to be too small a ratio for vehicles in the 10-40 MPG range for it to be easily understood by the general population. Gallons per 10,000 miles is too large a number, and similarly is difficult. I propose GPCM (gallons per 100 miles, C being the roman numeral for 100) as the standard. The numbers are small and easily understood, as the average travel per work day is near 60 miles? That could be totally wrong, but whatever, the figures you get from thinking about mileage in this framework are easy for everyone to understand, and easy to fit in forms/convert from existing numbers. It may also be easier for testing agencies to design tests for GPCM. I've thought about this a lot, and I'm more excited about it than anyone to whom I have described this idea. Good idea/bad idea? I'm open to whatever.
kwh per dollar would be more meaningful
written by Bruce Hamilton , July 20, 2008
Looking at the larger picture where you want to compare the efficiency of different fuels, all this "per gallon" (or liter) stuff is pretty meaningless, because different fuels have different amounts of energy per volume. So it seems to me that something like BTU per dollar or kilowatt-hours per dollar would be a much better general measure of fuel efficiency.
If you can't do the math...
written by theirritablearchitect , July 31, 2008
I guess doing more than moving a decimal around is too much work or something.
| ||
| < Prev | Next > |
|---|
Science, technology gadgets and...baby seals. We're in a bit of an eco-mess, but we've got the brains to lick any problem. And that's why EcoGeek.org publishes up to ten stories daily about innovations that are saving the planet.
And if that sounds interesting to you, then congratulations, you're an EcoGeek.
We tend to have bad measurement systems here in the US. We've got a temperature scale where water freezes at 32 degrees and boils at 212 (it's zero and 100 for the rest of the world.) And none of our measurements are compatible with our base10 numbering system. Twelve inches in a foot? 5280 feet in a mile? I mean...come on...
Subscribe to this comment's feed
And comparing how far two cars can go on one gallon of fuel seems easier to understand, because you can put a gallon of fuel in your car, but comparing over 10'000 miles, who drives that distance in one go?