On another note, I'm in the process of making just such an item. Interfacing with OBD2, pulling air flow data, temp's, rpm, speeds, etc. and calculating fuel mileage. The only problem is that nobody sells a blank PCB to stuff parts into. That's what I'm designing right now, getting the bugs out, making sure it can stand up to a vehicle's rotten power system. Hold off for another week or so and I should have a load of blank PCBs on hand.
JDG