
Miles per Gallon - ColinWright
http://www.solipsys.co.uk/new/MilesPerGallon.html?HN_20150507
======
markild
The illustration at the bottom of this[0] xkcd "what if?" illustrates this
pretty well.

[0]: [http://what-if.xkcd.com/11/](http://what-if.xkcd.com/11/)

~~~
ColinWright
Yes, I've been meaning to add that link at the bottom of the page with a short
piece of text for context - I'll do that soon.

Thanks.

 _Edit: Done that now. Not sure why this comment got a downvote, though._

 _Extra Edit: Hmm, now been re-upvoted. Bizarre, but thanks for the votes! I
know that they 're not actually worth anything, but they do indicate that what
I've done is appreciated, and considered to be of value._

~~~
ibmthrowaway271
Vote cancellation is weird.

I wonder if posts that have no genuine reason to be downvoted (like yours) get
more votes if they add a comment to the effect "Not sure why this comment got
a downvote" since multiple people will try and correct the erroneous downvote
with no idea whether anyone else already has.

Edit: Ha! Thanks for the downvote.

~~~
ColinWright
It is odd. When there's a strange downvote people might then make more upvotes
because they can see it was downvoted, but can't see when it's been corrected,
because the scores are no longer visible. But voting is weird anyway. I don't
take it personally, and I don't want to complain, but I genuinely get confused
when there are downvotes I don't understand, and I want explanations purely so
I can decide either that I was right and to hell with it, or I was wrong and I
need to reconsider things.

With just a downvote I don't get the chance to learn.

Still, there we are.

~~~
Svenstaro
Conversely, if people could just upvote outright but to downvote would need to
provide a minimum 10 character reason that was made public, would this break
of improve the system?

~~~
QuercusMax
You're describing something like Slashdot's moderation system, and we all know
how well _that_ worked out.

~~~
sukilot
At +5, It is the best moderation system ever published. Immune to brigades,
and reader-tunable to their individual preference level for jokes and flames.

------
perakojotgenije
I will never understand why Americans keep using miles per gallon instead of
liters per 100 km. Miles per gallon is only good if you want to advertise your
car ("look, with this car you can travel 50 miles with one gallon whereas with
that one you can only travel 40 miles").

But you never do that in real life, you never fill your car with say, two
gallons and say "now I'll travel 80 miles".

Instead you always know the distance between two points and you want to know
how much will that travel cost you. So you think like this: "distance from
point A to point B is 250km, my car uses 6 liters per 100 km so I'll use 15
liters, 1 liter price is 2€ so it'll cost me 30€". Easy as pie and you can't
do that with miles per gallon.

~~~
maxerickson
The thing about mpg that is problematic is that it obfuscates the savings in
going from a 10-20 mpg vehicle to a 20-30 mpg vehicle (and exaggerates the
benefits of a 50 mpg vehicle).

15 mpg -> 25 mpg is a much bigger step than 25 mpg -> 35 mpg (a savings of 2.6
gallons per 100 miles vs a savings of 1.15 gallons per 100 miles). The jump
from 35 mpg to 50 mpg only saves 0.85 gallons per 100 miles.

For everything else it just introduces some trivial difference in the
arithmetic.

~~~
dragonwriter
> The thing about mpg that is problematic is that it obfuscates the savings in
> going from a 10-20 mpg vehicle to a 20-30 mpg vehicle (and exaggerates the
> benefits of a 50 mpg vehicle).

I think the EPA's estimated-fuel-cost-per-year (based on standardized
assumption), which I believe is part of the standard rating disclosure for new
vehicles just like MPG is, addresses for most consumers that _better_ than MPG
or gal/100mi, and for the subset of consumers for whom it _isn 't_ better --
i.e., the ones that have good numbers on their own usage patterns that they
are going to calculate out from to get a personalized estimate -- MPG and
gal/100mi are exactly equal in utility.

~~~
maxerickson
I haven't shopped for a new car so didn't realize that they had changed the
labeling. The motivation for the cost estimates is clear enough from their
literature:

[http://www.fueleconomy.gov/feg/label/learn-more-gasoline-
lab...](http://www.fueleconomy.gov/feg/label/learn-more-gasoline-
label.shtml#you-save-spend-more)

(If you have a tiny screen, make sure to scroll down to the "MPG Illusion"
plot.)

Edit: You conclude: " i.e., the ones that have good numbers on their own usage
patterns that they are going to calculate out from to get a personalized
estimate -- MPG and gal/100mi are exactly equal in utility.".

What else did you take "For everything else it just introduces some trivial
difference in the arithmetic." to mean?

------
Serow225
As a car nut, this actually seems pretty reasonable; if you take that diameter
and divide it by (fuel injector duty cycle at cruise rpm, times number of
injectors), the resulting diameter should be pretty close to the car's
injector outlet effective diameter!

~~~
valarauca1
Hi, I actually work in fuel system analysis.

The one issue with the blog post that nobody seems to realize or point out. Is
that fuel consumption is measured, and calculated internally as kilograms per
kilometer. The change to volumetric units is done for the benefit of the
population.

This comes into play because namely, thermal expansion. Normally even in
return less engines fuel is used to cool injector heads, as well as some body
block parts (depending on engine design), as hotter fuel more readily reacts,
and is already being routed to the warmed parts of the engine.

------
bariumbitmap
This kind of calculation is trivial with GNU units:

    
    
        You have: 30 mpg
        You want: 1/(mm^2)
                * 12.754311
                / 0.078404861
    

It does reciprocal conversion automatically (12.75 mm^-2 corresponds to 0.0784
mm^2).

Note also that there are approximately 3.785 liters in a gallon, not 4.5461 as
stated in the original article.

~~~
dragonwriter
> Note also that there are approximately 3.785 liters in a gallon, not 4.5461
> as stated in the original article.

"gallon" is a unit of volume in several systems of measures, most notably the
US Customary system and the Imperial system (these systems are often
incorrectly conflated because many of the units are the same, and where the
units are different many of the _names_ are still the same.) The size of the
gallon is not the same in those systems:

    
    
      3.785 l/gal(US)
      4.5461 l/gal(Imperial)

~~~
bariumbitmap
Thanks, that makes a lot more sense.

    
    
      You have: 30 miles/imperialgallon
      You want: 1/(mm^2)
      	* 10.620186
      	/ 0.094160312

------
logfromblammo
If you start with mass per distance (kg/km), and you divide by fuel density
(mass per volume, kg/L or g/mL), you get area (ca or m^2).

If you start with distance per volume (km/L) and cancel the matching units,
you get inverse area (ca^-1 or m^-2).

This doesn't make sense until you realize it is the conversion factor between
volumetric flow rate and volumetric flux, and is the number you need to
associate volumetric fuel consumption rate (volume per time) with speed
(distance per time), which is exactly what you're doing when you think of the
fuel supply as a continuous trough.

This is a simplification. Different vehicle speeds will be more or less
efficient. Volumetric flow rate is an integral of the volumetric flux over an
area. So the area is the average aperture size. If your vehicle had a mass-
ignoring, initially-empty reserve tank of infinite size, scooping up fuel from
an infinitely long trough with the specified cross sectional area, it could go
an infinite distance. You need the tank because different speeds may require
more or less fuel flux. You need to travel at a more efficient speed before
you can go to a less efficient speed.

------
mark-r
You could do even better if the tube were carrying electricity instead of gas.
Maybe it would have enough power to carry a whole bunch of people at once?
Hey, I think I'm on to something!

~~~
ableal
(0.1 mm^2 is half the cross-section of AWG 24, getting close to hair thin.)

I don't think you're correct here. For round numbers, let's take a 100 kW
engine (134 hp), and suppose we feed 100 V. At full power, we'd be running
1000 Amps down that wire. Even at 1 kV it would take 100 A.

Better get a superconductor. Or a much thicker power cable.

~~~
pdx
He was using the wire AWG as a way to visualize the size for those of us that
are familiar with human hair, but unfamiliar with visualizing things that are
fractions of a mm thick.

He was not saying anything about replacing the amazing energy density of
gasoline with electricity over a copper wire.

------
userbinator
Incidentally, the volume of air required to ideally burn a litre of gasoline
is also quite large.

[http://en.wikipedia.org/wiki/Gasoline#Energy_content](http://en.wikipedia.org/wiki/Gasoline#Energy_content)

 _1 kg of fuel reacts with 3.51 kg of oxygen to produce 3.09 kg of carbon
dioxide and 1.42 kg of water_

Scaling that to the standard density of 0.755kg/L, that means 2.65kg of oxygen
for 1L of gasoline. Assuming the typical 20% oxygen content of air at standard
temperature and pressure, that is over 9000L of air.

So the air consumption of a car with fuel consumption of 0.1mm^2 is 0.0009m^2
or 900mm^2.

Edit: 900mm^2, not 900000.

~~~
lmilcin
If one needs 9000L of air to burn 1L of fuel, that would be 9000 _(0.1mm^2)
not (9000_ 0.1mm)^2. 900mm^2 is much saner don't you think. I doubt the car
would be able to scoop 900000mm^2 as it is 0,9m2. Can you imagine that amount
of air fed into engine? Me neither.

~~~
userbinator
Yes, you're right - I somehow thought 1L was 1m^3, when it should be 1L =
0.001m^3.

~~~
hso9791
It's not a strange assumption to make, given how the prefix system works for
true SI units. The litre/liter is defined as a decimeter cubed, or 0,1 m^3. It
is a metric unit, but not SI. Still, it is allowed within the SI system...

------
mdellabitta
Of course, you'd probably do a little better than that, because at no point
would your car be carrying all of that gas. But the trough would have to get
wider when you went uphill or had to get going at a light.

~~~
DougBTX
Instead of scooping up the fuel, you could think of it as laying out the fuel
in a line behind the car. All the fuel starts off in the fuel tank, and slowly
drains out of the car. Fuel efficiency is how large the hole is that the fuel
drains out of. In reality, it gets turned into a gas and blows away, but the
effect is much the same.

------
rm999
For comparison, what would be the radius of a continuous cylindrical battery
that powers the Tesla Model S? Assume, perhaps, that the battery has the same
energy density as a typical AA battery.

~~~
TheLoneWolfling
The Model S has an EPA rating of ~237.5 Wh/km. I also see that the Model S has
a volumetric energy density of ~700Wh/l.

Cancelling, I get ~0.34 mm^2.

Another question would be the mass per unit length. At 250 Wh/kg, that works
out to ~0.95g/m. For comparison, the 30mpg car works out to ~0.057g/m.

------
ableal
Using liters/100 km, it's a tad easier, we just have to get the orders of
magnitude right

\- liter = (100 mm)^3 = 1e6 mm^3

\- 100 km = 1e8 mm

So, 10L/100 km is 0.1 mm^2.

(Note that we want low values, and this is on the high side for European cars
- the usual advertised numbers for sedans are half of that, with reality
somewhere in between.)

------
samolang
How big of a solar panel would you have to stick on top of an electric car to
keep it going forever (ignoring the extra weight/wind resistance)?

~~~
QuercusMax
I suppose it depends on your definition of "car", but they've been doing this
for many years in the World Solar Challenge. GM's Sunraycer drove across
Australia in 1987 on solar power alone:
[http://en.wikipedia.org/wiki/Sunraycer](http://en.wikipedia.org/wiki/Sunraycer).

~~~
samolang
Yeah, I meant more like a Tesla or Chevy Volt.

