It plugs into the OBD2 port and reports what the ECU is seeing.
Real time readouts (~2 sec update):
speed, rpm, mpg, water temp, intake air temp, fuel consumption rate, closed/open loop, fuel pressure, ignition timing, engine load, manifold pressure, throttle position, voltage.
Haven't used them all, yet, but some stuff isn't reported by my ECU, like fuel pressure, so no data.
Also has a trip mode that reports:
max speed, max water temp, max rpm, average speed, average mpg, miles, time, fuel used
Has some setups that can show for current tankful:
fuel used, fuel remaining, distance, distance remaining, time, time remaining
Also acts as a scan/reset tool. Already used it to diagnose a bad o2 sensor. Check out the website, a pdf manual there, too:
http://www.scangauge.com/
Amazon has some resellers for a few bucks less. Worth it to me.
ScottB