Thanks a lot Sean and retepsnikrep! Lots of useful info there - probably more than I need. For now all I want to do is read MPH and display it - I hate analog gauges on vehicles!

In looking at the Scantool.net site, I think I can just get the ElmScan 5 Compact Scan Tool and use the serial port on a PIC to connect to it. Saves hacking the ELM chip.