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Miles per Gallon versus Liters per 100 Kilometers

112 points| tokenadult | 14 years ago |skepticblog.org | reply

87 comments

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[+] pash|14 years ago|reply
A poorly written post on a subject that is surprisingly intriguing when better elucidated. A study published in Science [0] several years ago makes for good discussion. That study found a systematic misinterpretation of the miles-per-gallon metric in the sense that participants consistently overvalued vehicles with high MPG ratings. They assigned values linear in MPG rather than linear in its inverse.

The study's authors told participants to "assume you drive 10,000 miles per year for work, and this total amount cannot be changed." The participants were then asked to come up with values for vehicles of varying fuel-efficiencies. That is indeed the sort of optimization problem people face when choosing which car to buy, and clearly a fuel-efficiency metric that puts the amount of fuel in the numerator makes the problem easier to solve [1] because expenditure is proportional to amount of fuel if distance driven is taken as given.

But in reading the article, I was struck by the lack of attention paid to the "miles" part of the equation. I don't fault the authors, but taking distance driven as fixed is surely an enormous detriment to the goal of reducing carbon emissions in America. Yes, reordering your daily life to drive fewer miles is more disruptive than simply buying a more car that goes farther on a gallon of gas. And, granted, once you've chosen your lifestyle, minimizing the amount of gas you burn as you go about your daily routine is the thing to do. All the same, it's ludicrous to ignore the basic inefficiency of the suburban style of life that dominates in this country while we wait for automotive engineers to come up with clever solutions to pricey gas and carbon emissions that are twice as high per capita as in many similarly wealthy countries. Surely living closer to where you work, using mass transit, biking, and walking more must be part of the solution as well.

Maybe houses and apartments should come with a "miles per day" multiplier denoting how far you'd typically travel getting to and from shops, restaurants, entertainment venues, and your place of work each day you live there. ...

0. http://nsm.uh.edu/~dgraur/niv/theMPGIllusion.pdf [PDF]

1. Apparently taking reciprocals is beyond the capabilities of most people. Or at least of the Duke undergrads who participated in the study.

[+] alexchamberlain|14 years ago|reply
The solution to global warming and pollution does not lie with persuading people to change, but making technological breakthroughs which allow us to carry on enjoying our lifestyles for a much lower cost to our enviroment.
[+] sliverstorm|14 years ago|reply
All the same, it's ludicrous to ignore the basic inefficiency of the suburban style of life

The trouble is an individual often has little power to do anything about it.

[+] kevinpet|14 years ago|reply
I don't understand the hate for this post. If you're shopping for a car, you're trying to balance factors like comfort, carrying capacity, impressing coworkers, purchase cost, resale value, maintenance costs, and of course, fuel costs.

Using a MPG measure would lead one to think that 20 mpg and 30 mpg are equidistant from 25 mpg, when in fact, going from 20 mpg to 25 mpg saves you $4,000 over 100,000 miles at $4/gal, while going from 25 mpg to 30 mpg only saves you an additional $3,667.

This has political implications as well, since CAFE standards are based on this non-linear value, a car company with a barbell of an extremely inefficient car and an extremely efficient car can have a better average than a company with a moderately efficient car even though the barbell uses more gas and pollutes more. (We'll ignore the irrational averaging by models rather than by sales -- that's another issue.)

I recently started looking at whether it would be worth it to replace my current gas guzzler and this was part of the calculation. You can see my numbers here: https://docs.google.com/spreadsheet/ccc?key=0AkSgUqAZJOp-dEJ...

[+] ta12121|14 years ago|reply
The hate comes from OP's description of MPGs as clearly inferior to L/100km when in reality he's just engaging in a Holy War between two things that are nearly equivalent.

The non-linearity argument is not as strong and you and OP think, because many people naturally think in ratios. In fact, in some anthropological studies hunter gatherers that don't know arithmetic mostly said that the value halfway between one and nine is three.

[+] schiffern|14 years ago|reply
In computing CAFE numbers, the inverse of mpg is used in the average. They are also weighted by sales, so both your points are specious.
[+] damoncali|14 years ago|reply
I don't understand the hate for this post.

It's because it's much writing and reasoning about comparing two ways to describe exactly the same thing, and comes to the conclusion that there is somehow a meaningful difference between them.

No matter how you slice it, you have to do some third grade math to figure out the economic impact of fuel economy. This is not a big deal.

It's the same faulty logic behind the supposed superiority of the metric system. Big whoop. You can multiply by 10. It's literally two sides of the same ruler.

[+] blahedo|14 years ago|reply
What an incredibly poorly reasoned post. First of all, the complaint has nothing whatsoever to do with the metric vs non-metric issue---we could as easily report gallons per hundred miles if it mattered that much.

More importantly, though, this business about the scale being "linear" is at best a red herring: linear with respect to what? And who cares if you save "one gallon" on a hundred mile trip? It's much more relevant how much you save relative to your overall gas budget, and saving one gallon out of three is much better than saving one gallon out of every seven.

And, surprise surprise, either system will tell us that just fine. Going from 14 mpg to 17 mpg may save about a gallon on a 100 mile trip (slightly more, actually), but it cuts your gas budget to 14/17 of its former amount, a savings of about 18%. And going from 33 mpg to 50 mpg may save a gallon on a 100 mile trip, but it cuts your gas budget to 33/50 of its former amount, a savings of about 33%. So, just as our intuitions would indicate, that's a much better improvement!

[+] btilly|14 years ago|reply
I disagree.

I think that most people think about gas in the following way (I know that I do). I commute distance x a day to work, how much am I going to pay in gas?

The 100kms/liter figure makes this straightforward. I can figure out how many km I'll drive in a typical year, multiply by that figure, and I have how many liters. Multiply by the cost of gas, and I have how much I have out of pocket.

With mpg I have to do a division step in there instead of multiplication. Division is a more complex operation. Laugh if you will, but people are more likely to screw it up.

[+] frankus|14 years ago|reply
MPG comparisons are pretty intuitive, but where MPG comparisons fall apart is when you're comparing comparisons.

Let's say you have a 10-MPG truck and a 50-MPG hybrid, and you drive both equally far each year, and you've got a $40k budget to replace one or the other.

Should you spend your money on a new truck that gets 11 mpg or a whiz-bang new EREV that gets 100 mpg (equivalent)?

Most people would think that improving the miles-per-gallon by 200% saves much more fuel than improving it by about 10%, but in this case both replacements save the same amount of fuel per mile driven.

If you invert both numbers it's immediately clear that this is the case (10 versus 9 gal/100mi, 2 versus 1 gal/100mi).

[+] stephengillie|14 years ago|reply
First of all, the complaint has nothing whatsoever to do with the metric vs non-metric issue---we could as easily report gallons per hundred miles if it mattered that much.

Exactly.

The measurement of gallons per 100 miles shows a much clearer picture of gas usage:

  MPG = gallons per 100 miles (100/MPG)
  14 = 7.1
  17 = 5.9
  33 = 3.0
  50 = 2.0

  MPG = Cost of gasoline to drive 15,000 miles at $4/gal (15000/MPG*$4)
  14 = $4285.71
  17 = $3529.41
  33 = $1818.18
  50 = $1200.00
[+] leif|14 years ago|reply
Linear with respect to your budget. The post is correct, that putting consumption on top gives a better "feel" for how much money you'll save.
[+] ams6110|14 years ago|reply
In practical terms it's not that clear cut. Let's say I have an old car that gets 14 mpg and I'm considering two new cars, one that gets 33 and one that gets 50 mpg.

If I drive 500 miles a week, either vehicle is going to save me at least 20 gallons a week in fuel. But going from the 33 to the 50 is only going to save me 5 more gallons. However to get 50 mpg I probably need to buy an expensive new hybrid, whereas it's not too hard to find a good used car that will get 33 for a fraction of the new hybrid's purchase price. Then you have to consider cost of financing and insurance (both more for the new car). If I go for the 50 MPG car I'll probably never make up the difference in fuel savings unless I drive a LOT of miles.

[+] typicalrunt|14 years ago|reply
When we talk about a car’s fuel economy, what we want to know is how much fuel does it use, not how far does it go

I've never understood this phenomenon. I realize he's talking about car economy here, but for me a car is a vehicle to take me from point A to point B, which means my most important metric is how far it goes, not how much fuel it consumes. Once I'm home and can (or want to) run calculations on my costs, then I'd like to know how much fuel it consumes.

[+] izak30|14 years ago|reply
It's a misnomer, sometimes you want one, and sometimes you want the other, so knowing a ratio, any ratio, and 8th grade algebra, you'll be fine. This is a silly post.
[+] jkrasnay|14 years ago|reply
I think that's his point. Usually you know how far you want to go and the question is, how much gas will it take. For example, I know my car gets 8 L/100km. If I want to go 400kms, I can quickly figure out that's 32L, or about 2/3s of my 48L tank. Doing this calculation with mpg requires division.
[+] rytis|14 years ago|reply
I don't understand what people are moaning about - it's well written and has valid points. To me MPG is like Kg/£ price labels - not that intuative at all.
[+] frankus|14 years ago|reply
Measuring fuel economy in miles per gallon is like setting up a budget in months per thousand dollars.

Big improvements to inconsequential expenses would result in huge mpkd improvements, but they wouldn't save you a lot of money.

[+] lotharbot|14 years ago|reply
> "setting up a budget in months per thousand dollars"

A lot of people actually do that: given the amount of money they have saved up, they figure out how many months they could get by without a paycheck.

Big improvements to inconsequential expenses aren't going to help you save a lot more money if you're gainfully employed, but they can help you last a lot longer if you're unemployed. Huge mpkd improvements matter in that situation.

Similarly, with the MPG or GP100M question, use the measurement that's most convenient for answering the question at hand. If you want to know how much fuel/money it'll take to go to Chicago, use GP100M. If you want to know what destinations are within reach for a certain amount of fuel/money, use MPG.

[+] abruzzi|14 years ago|reply
This probably doesn't impact that many people, but on a motorcycle with no fuel gauge, you use the odometer as your fuel gauge. You need to know how many miles to reserve, and how many miles to empty. So I regularly monitor how many miles to go from a full tank to reserve. While quantity per distance may be an easier metric at purchase time, that only happens once every few years. Distance per quantity is a calculation I do in my head several times a week to ensure that my vehicle maintains a good state of tune.
[+] NLips|14 years ago|reply
If you're using the odometer as a fuel gauge, surely volume becomes irrelevant for judging your range (excluding the tuning purposes)? e.g. My tank lasts 200 miles => I've gone 100 miles => I can go another 100 (top) before refilling.
[+] elchief|14 years ago|reply
Marginally related at best, but I only recently realized that a Quart is a quart-er of a Gallon. And a Liter is also a quarter of a Gallon (approximately). So basically a Liter is a Quart.
[+] Aloisius|14 years ago|reply
In the US, the dealer stickers are standardized by the EPA and DOT and contain annual fuel costs (either electricity or gas) based on 15,000 miles/year so you can make an easier comparison.

It also contains gallons per 100 mile ratings for gas cars and KwH per 100 miles for electric. I know it is shocking it can be done in imperial units, but somehow we managed.

[+] Drbble|14 years ago|reply
That's only at the dealership though, not in most marketing and conversations.
[+] duckduckgouser|14 years ago|reply
The biggest problem with miles per gallon as advertised is that it is inaccurate. Having every vehicle be required to provide that information without having to buy a separate device should be required by law.

And every vehicle should keep track of the cost required for the gas added to it, and then both provide stats on gas usage in volume/day, mpg (or similar), and show an average of how much it is costing you per day to use it, perhaps pointing out things you are doing that waste gas, like you ran the air conditioner with the windows down, your tires are underinflated, you need a tune-up, etc.

The U.S. says it wants to curb unnecessary energy usage, but instead of making laws that would actually help its citizens in this regard, it wastes money refunding money spent on ridiculously overpriced cars and bails out auto companies that were destined to fail. There are good reasons why it does this, but it's crappy nonetheless.

[+] zeroonetwothree|14 years ago|reply
Usually I care about how far I can go in my car before I need to refuel. This is exactly what MPG tells me.

The only time the inverse is useful is when buying cars on the basis of fuel efficiency. That's a much rare occasion since I drive every day but only buy cars every ~5 years.

[+] Derbasti|14 years ago|reply
But that is mostly a function of the size of your gas tank and not so much about fuel efficiency.
[+] therandomguy|14 years ago|reply
"When we talk about a car’s fuel economy, what we want to know is how much fuel does it use, not how far does it go."

Not sure about that. For Internet speed would you prefer to know how many seconds it takes to download 100MB as opposed to Mbps?

[+] Drbble|14 years ago|reply
Depends if you have a fixed MB demand or a fixed time budget, doesn't it? Which is better when you are downloading a 4.6G ISO?
[+] linuxhansl|14 years ago|reply
When we talk about a car’s fuel economy, what we want to know is how much fuel does it use, not how far does it go.

Now I grew up with the metric system and I would not generally agree with that statement. The function of a car is to transport me, so its output is distance. Measuring the output I get, rather than how much a standard unit of output would cost, makes sense to me.

[+] Drbble|14 years ago|reply
Do you drive as far as you can on a hundred euro note, or do drive to your destinations and then refill your spent petrol? Unless you enjoy cruising in your car with no time limits, it's the latter.
[+] rnadna|14 years ago|reply
One more thing: there are multiple definitions of "gallon". A lot of Canadians live near the USA border, and many cross the border to buy low-tax gasoline. The volume of a "gallon" differs by nearly 20% between the two countries.
[+] nraynaud|14 years ago|reply
for the Americans who might wonder : 100km -> 1h of driving on the highway (limit is 120km/h or 130km/h, remove urban part on start and finish and you're at 100km/h average).
[+] slantyyz|14 years ago|reply
>> 100km -> 1h of driving on the highway (limit is 120km/h or 130km/h, remove urban part on start and finish and you're at 100km/h average).

I find that to be a bit of a pointless observation. The speed limit that's on the sign on the highway is already a pretty decent indicator of how far you'll get in an hour, whether you're using metric or imperial.

[+] swah|14 years ago|reply
Actually, in Brazil at least, everyone thinks about fuel usage in km/liter (normally with two measures for city and roads).
[+] michaelochurch|14 years ago|reply
First, gas in the U.S. has been ridiculously cheap for decades. Housing, healthcare, and education costs are utterly teabagged, but gas is cheap, even now at $4. People who whine about gas prices are either (a) typical Boomer Republicans throwing their sense of entitlement around-- you know, the assholes who thought it was okay to go to war over "price gouging" in oil but clapped their fat hands together when their house prices octupled in one generation and young families got robbed (by them)-- or (b) people who live too ungodly far from where they work and are paying far more in lost time than in gas. Commuting (although less so by subway and not at all by bike) is corrosive and it has nothing to do with fuel costs.

Because gas is so cheap, people don't usually think in terms of, "how much gas will I use to get there?" Instead, it's "How far can I drive until I have to fill up?" That's a question that's easier to answer with a miles-per-gallon number.

Still, I agree that it would be better to see the inverse posted. Maybe people would think twice about driving those enormities. Probably not, but one can hope.

[+] greedo|14 years ago|reply
Aside from the ad hominem attacks, I think you're factually wrong. The reason people get upset about jumps in gas prices is because it directly affects the bottom line for a majority of Americans, whether they're Republicans or Democrats.

The median family income at the end of 2011 was about $50k. Take home would probably be around $3k/month. If you have a single earner driving 15K miles annually, in a car getting 25mpg, that's going to require 50 gallons of gas a month. So when gas jumps from $3 to $4, that's $50/month of take home pay that vanishes. For a two earner family, $100. Boom, there goes over 3% of take home pay.

For people with incomes below this, the percentage only rises. And they're not likely to have the latest Prius to minimize the impact.

In short, not everyone upset about higher fuel prices is a warmongering, fat-handed asshole who made out like a bandit during the housing bubble.

[+] minimax|14 years ago|reply
It's true that gas is cheap, but it's not true that people who whine about gas prices belong to any political or demographic group. It's basically a national past time here. It's like talking about the weather. It's part of our culture and it's something we'd all do at $0.50 / gallon or $5.00 / gallon.
[+] aioprisan|14 years ago|reply
yes, the relationships are inverted, but it doesn't make the relationship any less linear. also, the graph in the article is useless
[+] geuis|14 years ago|reply
Perhaps there's something here I'm missing, but this doesn't make sense to me at all: "Going from a 14 mpg car to a 17 saves exactly as much fuel (and carbon) as going from a 33 to a 50."

As the driver of a car, I want to go more miles on less gas.

If the car uses 1 gallon of gas to go 50 miles, and another uses 1 gallon to go 30, then it seems obvious that the first car is much more gas efficient. More miles on less fuel.

If car A gets 17 miles to the gallon, then it will take ~2.94 gallons to make 50 miles.

If car B gets 50 miles to the gallon, then it takes 1 gallon of gas to drive the same distance.

So comparing 13mpg to 17mpg and saying that its the same as the difference between 33mpg and 50mpg just doesn't make any sense at all.

[+] othermaciej|14 years ago|reply
Try doing the rest of the math.

33mpg -> ~1.52 gallons 50mpg -> 1 gallon for a 50 mile trip

So going from 33mpg to 50mpg saves half a gallon.

13mpg -> 3.84 gallons for a 50 mile trip 17mpg -> 2.94 gallons for a 50 mile trip

.9 gallons saved. So in your scenario, going from 13mpg to 17 saves more gas than switching from 33 to 50.

[+] jellicle|14 years ago|reply
MPG is actually much better for humans to use, as the end result is "bigger is better", which is how humans naturally think. It is unnatural to think "the smaller numbers are better". If you want it metric, it should be kilometers per liter.
[+] Drbble|14 years ago|reply
The people at the grocery store must love you! Bigger price? Sounds like Better price!