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Gas mileage is measured in square meters

 
Paul Clapham
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I never realized this before but it's true.

Read about it here: Droppings (there's some extraneous stuff at the beginning before he gets to the gas mileage topic).
 
fred rosenberger
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makes perfect sense to me.

except for the part why anyone cares about birds pooping.
 
Ryan McGuire
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Paul Clapham wrote:I never realized this before but it's true.

Read about it here: Droppings (there's some extraneous stuff at the beginning before he gets to the gas mileage topic).


Isn't it measured in inverse square meters? That xkcd page starts with gallons/mile, which might more appropriately be termed your car's "gas gallonage". Mileage is usually expressed as miles/gallon. i.e. length / volume or 1 / (length ^ 2)


Of course any inverse area unit would work. "How many inverse barns does your new Prius get? I'm still getting a little over 11 mega pascals per newton in my old car."

 
Paul Clapham
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Yes, well, most countries use the volume/distance method of measuring fuel efficiency.
 
Pat Farrell
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Paul Clapham wrote:Yes, well, most countries use the volume/distance method of measuring fuel efficiency.


Specifically something like Liters of fuel / 100 KM

Its actually a much better metric than the US's silly miles per gallon. The liters/distance is more linear so it matches our expectations better.
Human's don't understand the reciprocal well.
 
Pat Farrell
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BTW, my new VW TDI gets 49.9 mpg on real world trips back and forth between Washington DC and NYC
This while driving my usual 75 to 80
 
Mike Simmons
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Paul Clapham wrote:Yes, well, most countries use the volume/distance method of measuring fuel efficiency.

In that case, shouldn't it be called consumption, rather than milage or efficiency? Higher volume/distance -> higher consumption -> lower milage and efficiency.
 
Paul Clapham
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Mike Simmons wrote:
Paul Clapham wrote:Yes, well, most countries use the volume/distance method of measuring fuel efficiency.

In that case, shouldn't it be called consumption, rather than milage or efficiency? Higher volume/distance -> higher consumption -> lower milage and efficiency.


Not really, no. "Consumption" is a neutral term whereas "efficiency" is a value-laden term. It's perfectly possible to have a metric of efficiency where a lower value corresponds to a higher level of efficiency.
 
Mike Simmons
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I agree it's possible, but it seems backwards to me, just in terms of how it's labeled. I guess as long as the units are included people can figure out what it really means. Not sure how neutral vs. value-laden terminology factors in though.
 
Ryan McGuire
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Paul Clapham wrote:
Not really, no. "Consumption" is a neutral term whereas "efficiency" is a value-laden term. It's perfectly possible to have a metric of efficiency where a lower value corresponds to a higher level of efficiency.


That last sentence makes no sense to me at all. Are you saying it's perfectly possible to have a situation where low efficiency corresponds to high efficiency? I can see metrics where low numbers are good. e.g. Insect parts per hamburger patty. However people have a good sense of what high and low numbers are (e.g. 10 is higher than 7), and I would argue that saying an efficiency of 7 is "higher" than an efficiency of 10 qualifies as "wrong".

It should be obvious from the term used for a given metric whether big numbers are good or bad. Efficiency and mileage sound good. Consumption and "insect part content" sound bad. There are even exceptions to that: if we're talking about microbes eating an off-shore oil spill, "consumption" sounds good and we therefore want big numbers. However, an "efficiency" measure where small numbers are good is obviously mislabeled. If small numbers are good, it's not "efficiency". (Maybe inefficiency.)
 
Paul Clapham
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There aren't any "units" of efficiency in my sentence. I was using (or copying) the traditional Ranch definition of "efficiency" which has the general meaning of making code run faster or use less memory without being very specific about it. You'll notice that using less memory or taking less time to run is considered to be more efficient. I don't see why anybody would consider that a mis-labelling.

 
Ryan McGuire
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...not to detract from the thesis of the originally cited xkcd article: Unit cancellation is weird.
 
Paul Clapham
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So if I have the dimensional analysis correct, my car's fuel consumption is 0.086 square millimetres.
 
Pat Farrell
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I wonder if it would make more sense if we used a metric about energy rather than volume. Everyone knows that not all fuels have the same number of BTUs (or joules) per gallon, and its the BTUs that you want to measure. A lot of folks would be surprised to learn that "high test" gasoline has less energy than low test, and that racing gas has even less energy than high-test. So as a minimum, you have to burn more race gas to get the same number of miles/kilometers to make up for its lower energy density.

A joule is a watt-second so we can break the basic XKCD concept.
 
Maneesh Godbole
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fred rosenberger wrote:...except for the part why anyone cares about birds pooping.

One word. Guano
 
Campbell Ritchie
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Paul Clapham wrote:I never realized this before but it's true.

Read about it here: Droppings (there's some extraneous stuff at the beginning before he gets to the gas mileage topic).
0.1mm²? That is probably similar to the area of the jet which injects fuel into the engine.
 
Vishal Shaw
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Paul Clapham wrote:I never realized this before but it's true.

Read about it here: Droppings (there's some extraneous stuff at the beginning before he gets to the gas mileage topic).


One thing's for sure , I am not going to try this out in practise.
 
Bert Bates
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Quasi - hijack...

Is it better to increase your pickup truck's MPG from 12 to 15, or your car's MPG from 30 to 50?
 
Jayesh A Lalwani
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Depends on which one you drive more, innit?
 
Ryan McGuire
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Bert Bates wrote:Quasi - hijack...

Is it better to increase your pickup truck's MPG from 12 to 15, or your car's MPG from 30 to 50?


I drive my car approximately 600 miles a month and my truck only about 60 miles a month on the weekends back and forth to Home Depot. If I upgrade the truck, I would save 1 gallon per month. If I upgrade the car, I'd save 8 gallons a month. That's easy... upgrade the car.

Followup question:
Would it better to decrease my truck's GPHM (gallons per hundred miles) from 8 1/3 to 6 2/3 or my car's GPHM from 3 1/3 to 2, given my driving habits above?

 
Bert Bates
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I thought I could assume a few givens...

Let's say you drive your truck and your car the same (typical) miles / year.

Ok, so what's "typical"? I don't know, maybe 10-20k?
 
Steve Luke
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Bert Bates wrote:I thought I could assume a few givens...

Let's say you drive your truck and your car the same (typical) miles / year...


Then the answer there is easier to express if you use the Gallons / Hundred Miles (like Ryan). The Truck saves you 12/3 g/100 miles, the Car saves you 11/3 g/100 miles. If you use them equally, you should optimize the Truck.

[edit]
But Ryan should still optimize the Car
 
Ryan McGuire
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Bert Bates wrote:I thought I could assume a few givens...

Let's say you drive your truck and your car the same (typical) miles / year.

Ok, so what's "typical"? I don't know, maybe 10-20k?


If you assume them then they aren't "givens". :-)

Nonetheless, my semi-smart-ass answer came pretty close to proving what I assume was your point anyhow: In order to determine the relative attractiveness of the two options, I had to calculate the gasoline consumption from the MPG ratings. The followup question only helps the cause: the truck upgrade from 8 1/3 to 6 2/3 is worth 1 2/3 GPHM, while the car upgrade from 3 1/3 to 2 saves only 1 1/3 GPHM. Obviously GPHM or GPM is more useful for this type of question than the inverse MPG.

To answer your question...
Let's say I drive 12k miles a year to make the math work out nicely. The truck would go from 1000 GPY (gallons per year) to 800 GPY, saving 200 GPY. The car would go from 400 GPY to 240 GPY,, saving 160 GPY. Upgrade the truck.

HOWEVER...
My point was that while the truck upgrade is better when all other parameters (such as driving distance) are kept the same, the fact that one vehicle gets low mileage but has a higher hauling capacity means that the parameters explicitly aren't the same. You have to take the actual situation into account. In the specific case of my personal driving habits, one would have to take the given mileage or consumption rates and do the math to get the changes in amount of gas consumed. You could either divide 60 miles by 12 MPG to get 50 gallons or you can multiply 8 1/3 GPHM by (60 miles / 100 ) to get 5 gallons. Either way, you have to do a calculation of similar complexity to arrive at the amount of gas I have to buy for each vehicle, so the difference between the two metrics is pretty much a moot point. Tomayto / tomahto.


 
Bert Bates
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I mostly bring it up because there are a gazillion pickup trucks in the US, and you almost never hear folks talking about getting them from 12 to 15 mpg and you hear a LOT about Prius's getting 50 mpg. So I'm just saying that upgrading trucks ought to get more press than it does.

(Full disclosure, I have a 7.3 liter diesel truck myself, but I only drive it when I have to haul large loads like hay or horses )
(BTW it gets about 14 mpg)
 
Ryan McGuire
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My OCD is just strong enough to force me to fix a typo in my previous post.
Ryan McGuire wrote:You could either divide 60 miles by 12 MPG to get 50 gallons or you can multiply 8 1/3 GPHM by (60 miles / 100 ) to get 5 gallons.


60 / 12 = 5, not 50.

I feel better now that I got that off my chest.

 
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