I was going through this DIY section and found in the thread DIY homemade MPG monitor. In this thread some of the folks there are making some componets (Connectors) downloading a program(some are free),and using a sound card from their PC(personal Computer) to monitor what is going on in real time with their vehicle. They are basically monitoring the ALDL(Assembly Line Data Link) using the OBD( on board diagnostic) system. (This is how I understand it)
I was totally lost. I found a site that has some great info and some links to free program software and some that you can buy. They do a great job explaining how it works. I have not read everything on the site and do not understand how it all works.
I honestly just look at my fuel gauge a lot. if you keep an eye on it enough you can see what techniques your car doesn't like. when you get on it, etc. It's not 100% accurate but I can get a pretty good idea.