

Miles per Gallon versus Liters per 100 Kilometers - tokenadult
http://www.skepticblog.org/2012/05/24/mpg-vs-l100km/

======
pash
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
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.

~~~
InclinedPlane
More than that, the key point of contention is the fact that several countries
with a total population of billions of people (China, India, Indonesia,
Pakistan, Brazil, and others) are going to transition into developed "first
world" economies in the 21st century. Currently there is no known technology
that would allow them to avoid heavy reliance on carbon based energy sources
while that transformation occurs. It's easy to sit on our 1st world thrones
and imagine that if only we drove our cars a bit less everything will be fine,
in reality the issue is about holding back people who are living in poverty,
who don't have access to modern medicine, who don't all have access to clean
water, etc. It puts into perspective the putative hazards of global warming to
weigh against the cost of holding half the population of Earth in poverty.

~~~
ttt_
You make a very good point. Taking Brazil in perspective, middle class is
growing rapidly and with better credit the automobile industry is heavily
targeting these potential car owners. Makes good sense economically and the
state has been granting tax isemptions periodically which boosts sales
significantly.

However, car models are still pretty much inneficient fuel consumption wise.
That I know of, there exists only one hybrid model avaiable and it's very
expensive. Ethanol's price also turned it into a non-option for flex fuel
models.

Also, collective transportation is decades lagging pretty much everywhere that
it's needed. This leads anyone that can afford it to purchase either a car or
motorcicle in order to have decent mobility. It also makes it a class
distinction thing, as in public transportation is a lower class problem.

On the long run, IMO things look bleak for Brazil in terms of urban mobility
and greenhouse emissions.

------
kevinpet
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...](https://docs.google.com/spreadsheet/ccc?key=0AkSgUqAZJOp-
dEJENjFUYXpqRHNBMkk2c0RPbXp6eGc)

~~~
schiffern
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.

~~~
usaar333
Correct. Indeed, they are taking the average of the mpg directly, just the
harmonic average (<http://en.wikipedia.org/wiki/Harmonic_mean>) rather than
the arithmetic one we are more familiar with.

------
blahedo
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!

~~~
stephengillie
_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

~~~
einhverfr
That's one question ,but another one is:

How far can I drive per $100?

When looking at the car in part for discretionary uses, that's a valid
question.....

~~~
masklinn
> That's one question ,but another one is:

That is a question I've never seen anybody ask in my whole life. There are
precious few people who drive for the sake of driving, for the _vast_ majority
driving is utility to go from point A to point B.

Therefore, why optimize for the significantly less common (almost irrelevant,
even) case of "I've got money burning my pocket, how far can I drive?"

~~~
lotharbot
"how far can I drive for this amount of $" is a question I've only heard a few
times. But I do often hear "how far can I drive on a tank" or "how far can I
drive on my remaining amount of fuel".

Perhaps the disconnect we see in this comment thread is between people who do
almost entirely city driving and people who drive long distances. If you only
ever drive a few miles at a time, you don't really care what sort of distance
you can manage on a tank, you just care how much it's going to cost you to go
to work every day (you'll probably convert the fuel/distance ratio into a
money/time ratio.) If you spend a lot of time driving across central Kansas,
_how often you have to stop for gas_ becomes a more significant consideration
(you'll probably convert distance/fuel into stops/time.)

~~~
NLips
How often you stop for gas is surely better covered by a "range" measurement
for the car? Maybe it's different in the states, but all the cars I've ever
been in show your remaining fuel as a fraction of a tank. In that case, volume
of fuel is completely irrelevant, so MPG vs liters/100km has not place. e.g.
My tank takes me 400 miles => I have half a tank left => I can go another 200
miles ...is exactly equivalent to... My tank takes me 600 km => I have half a
tank left => I can go another 300 km

(For the purposes of this post, 1 mile = 1500m. If the size of a gallon
changes with locale, why not a mile? :-p )

------
typicalrunt
_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.

~~~
jkrasnay
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.

~~~
jlarocco
I don't see the big deal.

If I know my car gets 40 miles per gallons and I need to go 120 miles, I can
quickly figure out that I'll need 3 gallons of fuel.

I can't imagine either measurement being hard to use for anybody who's made it
to high school.

They're two ways of measuring the exact same thing and neither one has an
advantage over the other. Although it would be nice if we used kilometers per
liters instead.

~~~
sopooneo
Equivalent unit systems absolutely can have advantages over one another for
particular contexts. I don't know which of these systems is most convenient
(and most immediately useful) for most people, but I find it reasonable that
there would be an objective winner by that measure.

------
rytis
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
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
> _"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.

~~~
orangecat
_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._

Right, and the first is way more common.

~~~
lotharbot
> _"the first is way more common."_

Is it?

Figuring out total fuel costs is the sort of calculation I run once in a
while. When I buy a new vehicle, or when I'm planning a long trip, I figure
out how much I can expect to spend in fuel. I'm almost always at a computer
when I make this sort of plan, though.

But when I'm actually driving and having to do calculations in my head, the
range calculation is far more useful. Am I going to have to stop for gas
between here and there? Should I fill up today or can I wait until tomorrow?
Can I make it to Colby or should I fill up in Goodland? This subject comes up
every time I take a medium length trip, and several times on a long trip, as
compared to the other calculation which comes up only rarely.

------
abruzzi
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
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
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
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
That's only at the dealership though, not in most marketing and conversations.

------
duckduckgouser
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
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
But that is mostly a function of the size of your gas tank and not so much
about fuel efficiency.

------
therandomguy
"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
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
_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
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
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
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
>> 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.

~~~
Drbble
Cool, but I don't read HN on the highway.

------
swah
Actually, in Brazil at least, everyone thinks about fuel usage in km/liter
(normally with two measures for city and roads).

------
michaelochurch
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
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.

------
aioprisan
yes, the relationships are inverted, but it doesn't make the relationship any
less linear. also, the graph in the article is useless

------
geuis
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
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.

~~~
geuis
If you're just comparing percentages, then yes 0.9 > 0.5.

However, overall the comparison doesn't stand up. 1 gallon for 50 miles vs
2.94 gallons, the 50 mpg is what is saving more gas, therefore it wins.

------
jellicle
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
The people at the grocery store must love you! Bigger price? Sounds like
Better price!

