I was searching for a pinout for the toyota diagnostic port on my 1994 geo prizm to see if I could get more than just the O2 sensor off of the connector, and I found a tool that might work.
primavistalab.com/myengine/ - Translator Down at the bottem of the page is the English version.
I'll test it when I have a chance to build the cable, but it looks promising. Looking at the translated version, the software can show fuel consumption. I know a MPGuino is a better long term solution for a FCD, but this allows me to see what the ECU is seeing so I can find any issues with my car.
EDIT: I found a PDF that outlines behavior in diagnostic mode, it looks like you can setup a micro-controller real easily to read the code.
http://carina-e.ru/download/file.php?id=20531
I found what looks to be the source code for the software, or it might not be, I can't tell, but it has the same results, it looks like what I found can be re skinned for English, gallons, and miles.
EDIT:
Cartester8000 seems to work for reading the data stream. A USB to RS232 dongle off of ebay and a simple cable is all that is needed hardware wise.
OBD-1 Serial Interface - Toyota Wiki