OBD Guage
 

Check out https://www.qcontinuum.org/obdgauge/. With one of the ~$100 OBD1 diagnostic port to serial converters, you can get a Palm Pilot or PC to datalog and display data. Unfortunately, it isn't calculating mileage, but the source code is there so if you know C, the program could me modified to do it.

I don't have an OBD2 car, but this kind of makes me wish I did.

me 2

Excuse me while I drool. I have both a Pocket PC and OBDII.

Now if I only had the extra $$$.