*reproduced here with permission*<br/>
 DIY MPG GAUGE using your laptop and a simple circuit. Now anyone who has a 
  laptop or tablet in their car can monitor MPG for pennies <br/>
<img src="Mpg.GIF"> <br/>
*tank will persist when window is closed <br/>
Using this circuit, with right and left switched<br/>
<img src="lap.GIF" width="396" height="227"><br/>
 Zoom in on recording of line in (44100 8 bit stereo) for anylizing the waveform 
  (vss* on top, injector on bottom) and general futzing in response. I don't think 
  the diodes are doing much on the vss side <br/>
<img src="graph.GIF" width="692" height="478"><br/>
*Yellow line is default threshold for the vss, <br/>
*Cyan "..." injector <br/>
*I adapted the code to the latest wave form (inj pulse was on other channel 
  and right side up?!?) <br/>
*vss is the misnamed Vehicle Speed Sensor. It really only measures distance.The 
  computer watching the vss still has to keep track of time to figure out the 
  velocity. <br/>
Speaking of code, here it is in it's entirety, 6 drama filled pages to chew 
  on in one file called <a href="Mpg.java">Mpg.java</a>  <br/>

&nbsp;<br/>
&nbsp;<br/>
So, believe it or not, it's all there. I think its a solid proof of concept 
  anyway. Every car will be different, and there's plenty else that can be done 
  to it: RPM column, invert signal/swap channel from mpg.properties file. A a 
  collection of presets for different cars... But I, personally, am gonna try 
  and do other stuff for a while <br/>

<br/>
Update, a nice install guide specific for a metro can be found <a href="http://www.teamswift.net/viewtopic.php?t=33317">here on teamswift.net</a>