
Why 'gallons per mile' is better than 'miles per gallon' - squeakynick
http://datagenetics.com/blog/april12014/index.html
======
amluto
I was hoping that an article about careful use of measurement units wouldn't
contain a blatant mathematical error.

The article calculates, presumably correctly, that driving 70mph costs
$3.66/hr more than driving 55mph in some reference car. It concludes that
driving 70mph is worthwhile if getting to your destination an hour early is
worth more than $3.66.

This is completely wrong. Driving 70mph for an hour gets you 15 miles farther
than driving 55mph for an hour. Doing that costs you less than $3.66, since
you're driving for less time. It also doesn't save you anywhere near an hour.

~~~
vlasev
I think that's as good of an example as any provided by the author as why the
US should do away with the mpg measurement. If even the careful author can
make a mistake like that, then think about the average person.

------
dredmorbius
MPG is useful for range estimates: if you've got n gallons, you can travel m
miles.

GPM is useful for budget estimates: if you have a trip of m miles, you'll need
n gallons of fuel, with some given cost.

Or you can use a tool such as GNU units which handles reciprocal conversions
with ease and aplomb.

~~~
gamegoblin
Never used GNU units, but Wolfram Alpha will also do this trivially [0].

[0]
[http://www.wolframalpha.com/input/?i=16+mpg+in+barrels%2Ffur...](http://www.wolframalpha.com/input/?i=16+mpg+in+barrels%2Ffurlong)

~~~
cbr
As will google:
[https://www.google.com/search?q=16+mpg+in+barrels%2Ffurlong](https://www.google.com/search?q=16+mpg+in+barrels%2Ffurlong)

------
SixSigma
The United Nations creating a uniform system of regulations, WP.29 was
established on June 1952, for vehicle design to facilitate international
trade.

[http://en.wikipedia.org/wiki/World_Forum_for_Harmonization_o...](http://en.wikipedia.org/wiki/World_Forum_for_Harmonization_of_Vehicle_Regulations)

As of 2012, the participants to the 1958 Agreement include 52 countries
including all of Europe, China, Russia, Japan, Australia.

Part of the regulations is a common specification for fuel use - litres per
100km.

There is one significant non-signatory to the agreement.

~~~
jaggederest
Could even use gallons per 250 miles as a fair approximation in US-style
units, and I think it's a more practical measure than gallons per mile since
it indicates perhaps a week of moderately heavy driving.

~~~
twic
Gallons per 280 miles is even closer. And since there are seven days in a
week, you can weakly justify that as 40 miles a day every day of the week.

------
werdnapk
Canada pretty much uses X litres/100km nowadays recently replacing mpg. The
lower the litres, the more efficient the vehicle. It actually was fairly easy
to become accustomed to these new metrics.

~~~
burke
It's an extremely convenient metric.

L/100km is roughly equivalent to L/h at highway speeds, and you can figure
Litres to destination when you see a road sign by multiplying L/100km by the
hundreds place.

With gas stations being upwards of a hundred kilometres apart in the prairies
and northern ontario, especially at night, it's nice that this is so easy.

~~~
blahedo
Ah, good point: so that means we should be using Gallons per 60 miles! :)

------
danbruc
Next: Why liters per 100 kilometers is better than gallons per mile.

~~~
pash
See
[https://news.ycombinator.com/item?id=4020885](https://news.ycombinator.com/item?id=4020885)
:P

~~~
notduncansmith
Well played, sir.

------
dnautics
> Fuel consumption is a better measure of a vehicles performance because it is
> a linear relationship with the fuel used, as opposed to fuel economy which
> has an inherent reciprocal distortion.

Fuel economy is a better measure of travel potential because it is a linear
relationship with the distance you can go, as opposed to fuel consumption,
which has an inherent reciprocal distortion.

~~~
RaptorJ
An interesting switch, but who cares? There are gas stations everywhere.

~~~
username223
Everywhere? Try driving across Nevada or southern Utah.

~~~
balls187
On the main roads, it looks like there are a decent number of fuel options,
within a 50 mile radius.

[https://www.google.com/maps/search/gas+stations+near+souther...](https://www.google.com/maps/search/gas+stations+near+southern+utah/@39.2742659,-111.6872373,7z)

~~~
lotharbot
I recently drove from Raton, New Mexico eastbound along US 87. There were a
lot of abandoned gas stations along that stretch. The first operational one
was 40 miles away in Des Moines; the next one was another 40 miles away in
Clayton.

Sometimes that sort of spacing makes the difference between one stop and two,
or two stops and three, on a longish drive. Or, put another way, another 20
miles of range makes the difference between filling up in this town or filling
up 40 miles down the road.

~~~
balls187
Once during a cross country trip, I decided to not fill up until I came across
the next station.

This was before GasBuddy and other smart apps that can help alleviate some of
these situations.

The result was rolling into a station on a Native Reservation which only
accepted cash.

------
anigbrowl
I'll settle for reciprocal distortion if more people would switch to the
metric system.

~~~
dredmorbius
Most metric countries report liters/100km, solving both problems at the same
time.

~~~
ProblemFactory
An useless but fun observation: you can do the litres/100km division and
arrive at _units of area_. A family car might have a fuel consumption of 0.05
mm^2.

This is the cross-section of the imaginary pipe of fuel along the highway that
a car with no fuel tank would have to pick up as it travels.

~~~
keule
[https://what-if.xkcd.com/11/](https://what-if.xkcd.com/11/)

------
pash
_This topic came up on HN a couple of years ago [0], and I posted a comment
that was well received. So here it is again, edited for the present context
and updated with my recent thoughts:_

People do seem to misinterpret MPG ratings. A study published in _Science_ in
2008 [1] found 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 just the sort of optimization problem people face when choosing which
car to buy, and apparently a fuel-efficiency metric that puts the amount of
fuel in the numerator makes the problem easier to solve because expenditure is
proportional to the amount of fuel burned, at least when distance driven is
taken as given.

But in reading many of the words expended on this topic over the last couple
of years, my lasting impression is that this a lot of hullabaloo about the
wrong problem. It's the lack of attention paid to the "miles" part of the
equation that most needs fixing. If the goal is to reduce carbon emissions (or
any of the other negative externalities of driving), then taking distance
driven as fixed frames the problem in a way that obscures the real solution:
we should be encouraging people to drive less.

Yes, reordering your daily life to drive fewer miles is more disruptive than
simply buying a car that goes farther on a gallon of gas. And, granted, once
you've chosen your style of life, minimizing the amount of gas you burn as you
go about your daily routine is the thing to do (even if your optimization
problem is a purely financial one). All the same, it's ludicrous to ignore the
basic inefficiency of the suburban lifestyle that predominates in America
while we wait for automotive engineers to come up with clever solutions to
pricey gas and to carbon emissions that are twice as high per capita as in
similarly wealthy countries.

Elon Musk isn't going to save us all by himself. Surely living closer to where
you work, using mass transit, cycling, and walking more must be part of the
solution as well. ... So maybe houses and apartments should come with a "miles
per day" rating suggesting how far you'd travel getting to and from shops,
restaurants, entertainment venues, and your place of work every day you live
there. ...

0\.
[https://news.ycombinator.com/item?id=4020885](https://news.ycombinator.com/item?id=4020885)

1\.
[http://www.sciencemag.org/content/320/5883/1593.summary](http://www.sciencemag.org/content/320/5883/1593.summary)
(paywalled);
[http://nsmn1.uh.edu/dgraur/niv/theMPGIllusion.pdf](http://nsmn1.uh.edu/dgraur/niv/theMPGIllusion.pdf)
[PDF]

~~~
jseliger
Great comment. I'll add an observation:

 _we should be encouraging people to drive fewer miles_

To do this we basically need denser neighborhoods, as Edward Glaeser points
out in _The Triumph of the City_ ([http://www.amazon.com/Triumph-City-
Greatest-Invention-Health...](http://www.amazon.com/Triumph-City-Greatest-
Invention-
Healthier/dp/159420277X?ie=UTF8&tag=thstsst-20&linkCode=as2&camp=1789&creative=390957\)—but)
he points out, as does Matt Yglesias in _The Rent is Too Damn High_ , that the
big problems are with local zoning requirements, which by and large forbid
density increases.

There are lots of local battles going on regarding density, and I agree that
these are good things: "living closer to where you work, using mass transit,
biking, and walking more," but they can all be encourage or discouraged by
zoning. In most of America they're discouraged.

In the meantime better mileage is at least an improvement.

~~~
jimmaswell
A lot of people simply don't want to live in a crowded city in an apartment
crowded on every side with other apartments. I'd prefer a location in a low-
density suburban area (with a lot of forest around me) where I could get to
the city in a few minutes.

~~~
nowarninglabel
That's a false dichotomy. There are ways to create suburban areas with forests
and local gardens that are much higher density than our current suburban
outlay while still being enjoyable to live in. However, it requires giving up
the McMansion style of building. The only way for this to happen though, in my
opinion, is to incentivize developers into planning these types of
neighborhoods vs. McMansion lands (and I say this from an experienced
standpoint with a family of real-estate developers).

~~~
jimmaswell
You misunderstood. I meant literally around me. House surrounded by at least a
mile or a few miles of woods. If the houses were lined up in front of the
forest you could increase the density, though. But the way I posed it
originally, not so much.

What's your threshold for where a house becomes a "McMansion?" What would you
think of ~1200sqft, two stories with a basement, with a good yard?

~~~
baq
i think it's the quantity. it's not one house, it's that there's a
neighborhood with a thousand of very similar houses.

~~~
lttlrck
How is that alone a density issue?

~~~
Retric
The lower the neighborhood density the less incentive there is for local
retail. Without local retail everyone is forced to drive and in some cases
drive fairly long distances when the area is full of low density houses.

Still, the internet can significantly reduce peoples need to drive because
package delivery can be much more efficient than driving to a shopping mall.

------
bigbugbag
From a European point of view it seem obvious that the gallons per mile trick
is a small but effective cog in the larger US automobile conspiracy going on
for decades. From the destruction of public transportation[1] to the many
billions of dollars bailouts and current tesla battle this conspiracy is quite
an obvious and documented one.

[1]
[https://en.wikipedia.org/wiki/General_Motors_Streetcar_Consp...](https://en.wikipedia.org/wiki/General_Motors_Streetcar_Conspiracy)

------
analog31
I think there are other examples of this, such as price-to-earnings ratio, and
focal length of lenses. In those cases, since a denominator can go through
zero, the ratio goes through a big discontinuity. And I've seen p/e ratios
reported as "n/a" when they should really be represented by a big negative
number.

------
pbreit
Either I did not understand the post or it is totally uncompelling. MPG makes
much more sense to me. Gallons per 100 miles (or whatever) sounds very clumsy.

~~~
sopooneo
A typical person drives a fixed number of miles per month, rather than using a
fixed number of gallons of gas. So one advantage of "Gallons / 100 miles" is
that it requires one fewer steps of arithmetic to figure out how much gas you
need every month.

~~~
lotharbot
On the other hand, a typical person driving between Denver and New Orleans
knows they just put 14 gallons in the tank and would probably like to figure
out how far until their next required fuel stop.

Different use cases mean different natural measurements.

~~~
Vik1ng
It's not that bad then either.

If you have 3 gallons per 100 miles, then 3 fits into 14 a bit less than 5
times so you will get like 470 miles.

I mean it's not going to be exact anyway depending on the conditions.

~~~
lotharbot
right, I'm not saying one measurement is "bad" and the other is "good". Just
that one is more _natural_ for specific conditions.

Something I learned in grad school (applied mathematics): a lot of people care
about metric because the conversions are easy. But a lot of systems are better
measured in units that come from the system itself. Traffic flow may be best
measured, not in feet or meters, but in multiples of the average car length.
If you get too caught up in trying to force everything into metric just
because it's usually better, you can end up creating extra work for yourself
in circumstances where the system is better measured using itself. Likewise,
if you insist on gpm because it's "better" than mpg, you make some
calculations a little easier and others a little harder.

~~~
kilburn
> Traffic flow may be best measured, not in feet or meters, but in multiples
> of the average car length.

And you've just thrown out the other advantage of metric: universality. The
average car length is different in USA than in Europe. If you use "the USA
average" to count flow in there, and "the EU average" to count flow in Europe,
then you cannot compare those measurements afterwards. If you force EU to use
"the USA average car length", then the measure is extra counter-intuitive for
EU people. If you just use the "world average car length" then the figure is
subtly counter-intuitive for most people.

That's why it is better to stick with a universal measure, _and_ let that
measure be easy to operate with. The metric system provides both features.
Then just let people do what they're good at: figuring out what the "normal"
ranges of values are and work from there.

~~~
lotharbot
> _" you've just thrown out the other advantage of metric: universality"_

Nope. I've done the opposite.

Let me explain: when you set up a traffic flow equation (or many other complex
equations), you don't write it in terms of meters or feet or any other
supposedly universal unit. You write it in terms of whatever the natural scale
is -- whatever makes the math simplest. (You might define the average car
length as "1", for example.) Then once you've finished solving your equations,
you can rescale everything in whatever units are most familiar to the audience
you're presenting to.

------
sillysaurus3
Also time-per-frame instead of frames-per-second.

~~~
deletes
Frames per seconds is better in almost every way. When you use it to watch
movies it is better since a normal human understands seconds but not what a
frame is. In gaming, frame times usually vary quite a lot, and the average,
which is fps, is the important part.

~~~
ygra
When doing real-time graphics and looking at a profiler, it _does_ help to
know that you want to have less than 16 ms of calculations per frame. But
agreed, in most other respects fps is nicer.

------
return0
Well, "liters per kilometer" would be even better.

~~~
bigbugbag
I disagree. First, the human mind is not that good at dealing with decimal and
numbers between 0 and 1. My car is rated 4.5l / 100 km, which is easier to
deal with than 0.045l / km.

For a simple calculation, say 5km, what the easiest 0.045 * 5 or (4.5/10)/2 ?

Secondly, a car is designed to be driven over distance, so making measurement
on only one kilometer wouldn't be representative of real world usage.

~~~
return0
agree, anything but gallons or miles. L/Km is the standard in europe anyway.

------
shmerl
How about switching to the metric system altogether?

------
leccine
I guess this is why l / 100km is used in the EU instead of km / l.

~~~
Ecio78
Actually in Italy I've always seen _km per liter_ for years and years, let's
say all the 80s and 90s ("my car runs 14 km per liter, and yours?" "mine is
crap, I can't do more than 10km with a liter of gasoline" and same for auto
magazines). I think only recently (new century?) they started using more the
"liters per 100km" approach

------
taeric
Citations, or you are just promoting your opinion.

I'm also curious about the numbers showing that 55mph is the most efficient. I
do not particularly dispute that this is the case for the majority of
vehicles. I am curious as to whether or not this is as it has to be, or
because that is the way vehicles are built in the US. That is, could you do
better with different high speed gearing?

~~~
wodenokoto
> Citations, or you are just promoting your opinion.

This is an inherently flawed statement. If you follow the citations in a
scientific paper at some point, the citations are going to end. A scientific
article that consist only of citated information doesn't bring anything new to
the table, it only summarises.

~~~
baddox
I think most people, when requesting a citation in this context, would also be
fine with a description of an experiment (preferably a repeatable one) and a
record of the empirical result of that experiment. That's the "base case"
you're talking about in the otherwise infinite regression of citations.

