Randall has us covered, as always. http://xkcd.com/687/ http://xkcd.com/687/ and "You can even plug it into Wolfram|Alpha, and it’ll tell you that 20 MPG is

Randall has us covered, as always. http://xkcd.com/687/ http://xkcd.com/687/ and "You can even plug it into Wolfram|Alpha, and it’ll tell you that 20 MPG is about 0.1 square millimeters (roughly the area of two pixels on a computer screen)." https://what-if.xkcd.com/11/ https://what-if.xkcd.com/11/