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.