I've been lusting after National Instruments' USB-6008 DAQ, which is $170. 8 channels accepting -10V to 10V, 10kHz sample rate, 12b resolution, transmits data to a laptop in the car via USB. NI's LabView software allows you to translate those voltages back into temperatures, speeds, etc. in real time if you like.
Of course, NO sensors are included. However, most of the sensors you might want are already installed in the car. Others can be purchased seperately, such as:
Jameco Electronics Robotics: PARALLAX INC.: 28526 which could be epoxied to the frame of your car.
I would like to log wheel speed (and compute two differentials thereof), fuel injector #1 activity, HV battery current, ambient air temperature, upstream (wideband) O2 sensor output, and any other sesors relevant to whatever I'm working on.
At the very least, this would yield a lean-burn prediction algorithm, and give me a breakdown on where I spend energy on my commute. Not just at which mile-marker, but it should also give me an idea of whether it's due to aero, rolling, or braking.
What I really want is to break MPG into its constituent components: powertrain efficiency (BSFC) and energy consumption per mile (rolling, aero resistance, and braking, predominantly). In an ideal world, you'd have a gauge displaying each in your car.
I haven't done it yet because it's time consuming and expensive. But it would yield top-quality data, and lots of it.