By "plugs into your car's onboard computer" do you mean through the OBD port? If so, there are a few open source projects out there. Searching the usual OSS repositories may turn up a hardware engineer who's startup minded. There must be lots in Boston!

Here are a few search results to get you started:

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1222013546

http://hackaday.com/2009/05/12/obd-ii-automotive-data-loggin...

http://sourceforge.net/projects/obdgauge-plus/

http://github.com/benvm/garmon

A mix of hardware & software, but you can take it from there. Good luck!