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 09-12-2012, 11:43 AM #1 (permalink) Aero Deshi     Join Date: Jan 2010 Location: Vero Beach, FL Posts: 1,065 MagMetalCivic - '04 Honda Civic Sedan EX Last 3: 34.25 mpg (US) Thanks: 430 Thanked 668 Times in 357 Posts XKCD Interesting Factoid I found this funny thing on the xkcd.com web site Written by Randall Munroe and it is Okay to copy into here. (XKCD Copyright Info) My take away on this is that it is astounding that such a small amount of fuel squirts us around to where ever we're going. I have to say—from a dimensional analysis standpoint, ”poops” is one of the strangest units I’ve ever tried to cancel in an equation. But there’s another case of odd unit cancellation, common in everyday life, which is—in a way—even weirder: Gas mileage. In the US, we measure fuel economy in miles/gallon—which could just as easily be written as gallons/mile. (This reciprocal form has some advantages. It’s popular in Europe, where it’s expressed as liters per 100 kilometers.) But regardless of which units you use, there’s something strange going on here. Miles are units of length, and gallons are volume—which is length³. So gallons/mile is length³/length. That’s just length². Gas mileage is measured in square meters. You can even plug it into Wolfram|Alpha, and it’ll tell you that 23.5 MPG is 0.1 square millimeters (roughly the area of two pixels on a computer screen). Unit cancellation is weird. Ok, so what’s the physical interpretation of that number? Is there one? It turns out there is! If you took all the gas you burned on a trip and stretched it out into a thin tube along your route, 0.1 square millimeters would be the cross-sectional area of that tube. Here's the complete XKCD page.
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 09-12-2012, 01:03 PM #2 (permalink) Human Environmentalist     Join Date: Aug 2010 Location: Oregon Posts: 11,270 Acura TSX - '06 Acura TSX 90 day: 28.24 mpg (US) Lafawnda - '01 Honda CBR600 F4i 90 day: 47.32 mpg (US) Big Yeller - '98 Dodge Ram 2500 base 90 day: 21.82 mpg (US) Prius Plug-in - '12 Toyota Prius Plug-in 90 day: 57.64 mpg (US) Mazda CX-5 - '17 Mazda CX-5 Touring 90 day: 25.04 mpg (US) Chevy ZR-2 - '03 Chevrolet S10 ZR2 90 day: 17.14 mpg (US) Thanks: 3,833 Thanked 4,142 Times in 3,148 Posts Very interesting. I will be giving my fuel economy figures in terms of mm^2 from now on :P How efficient does my car need to be to achieve tubes that are as thin as a strand of hair? __________________ Gas and Electric Vehicle Cost of Ownership Calculator Give me absolute safety, or give me death!
09-12-2012, 02:32 PM   #3 (permalink)
Aero Deshi

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Quote:
 Originally Posted by redpoint5 Very interesting. I will be giving my fuel economy figures in terms of mm^2 from now on :P How efficient does my car need to be to achieve tubes that are as thin as a strand of hair?
According to Wikipedia, human hair can be 17 to 180 micrometers in diameter. So based on that, I figured you'd need to get between about 90 to 10,000 MPG depending on the color of, and location harvested, of the strand of hair you are using.

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 09-13-2012, 11:43 AM #4 (permalink) aero guerrilla     Join Date: Oct 2008 Location: Warsaw, Poland Posts: 3,665 Svietlana II - '13 Peugeot 308SW e-HDI 6sp 90 day: 58.1 mpg (US) Thanks: 1,219 Thanked 705 Times in 446 Posts Frontal area is also measured in sq. meters, so maybe the ratio of the two numbers would mean something? Like who gets the best FE per unit of FA. Or maybe it's a constant, an important building block of the physics of the Universe? __________________ e·co·mod·ding: the art of turning vehicles into what they should be What matters is where you're going, not how fast. "... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell [Old] Piwoslaw's Peugeot 307sw modding thread
09-13-2012, 10:06 PM   #5 (permalink)
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That's an interesting way of expressing fuel economy. My Mustang has a lifetime average of .07968 mm^2, or .0063" diameter - just over twice the diameter of an average human (scalp) hair.
mm^2 = (liters/100km)/100.
The amount of energy contained in petroleum hydrocarbons is quite impressive.
__________________
Fuel economy is nice, but sometimes I just gotta put the spurs to my pony!

Quote:
 Originally Posted by thatguitarguy Just 'cuz you can't do it, don't mean it can't be done...
Quote:
 Originally Posted by elhigh The presence of traffic is the single most complicating factor of hypermiling. I know what I'm going to do, it's contending with whatever the hell all these other people are going to do that makes things hard.

Last edited by Mustang Dave; 09-13-2012 at 10:12 PM..

09-14-2012, 08:23 AM   #6 (permalink)
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I saw that comic this morning, and at first I was: "funny", then... "what?" then... oooh... that's pretty logical. This one was almost as awesome as the half-empty glass.

Quote:
 Originally Posted by Piwoslaw Frontal area is also measured in sq. meters, so maybe the ratio of the two numbers would mean something? Like who gets the best FE per unit of FA. Or maybe it's a constant, an important building block of the physics of the Universe?
It would be interesting to find out what the theoretical maximum you could get for every square meter of frontal surface area is, and if there is a linear relationship.

 09-14-2012, 09:05 AM #7 (permalink) Super Lurker!     Join Date: Sep 2012 Location: Chesapeake, VA Posts: 88 Rusty - '88 Chevrolet S10 base 90 day: 23.72 mpg (US) Doc - '08 Honda Civic EX-L 90 day: 29.6 mpg (US) Thanks: 4 Thanked 14 Times in 11 Posts That really is a pretty cool way of looking at fuel economy. Oh and thanks for killing like an hour of my time with that... because of course I had to go back and see all of the other "what if" questions that were asked. The first one was absolutely hilarious BTW.