Don't most later model vehicles have this information available on board?
Ya - The ScanGauge just plugs in to your car's OBDII connector and lets you display whatever data's available that you want to see. Higher end versions of my truck have a display built in to the dash.
I had to set some codes to display the transmission temperature (The instructions are on their web site). Evidently its not available from from vehicles.
Gas consumption features:
When you first set it up you enter your vehicles gas tank capacity and when you do the next few fill ups, you correct the quantity of gas your car thinks you pumped in to calibrate the unit. I think its as accurate as I usually am doing division on the back of a gas reciept.
I like it for the four extra gauges I keep displayed - intake air, transmission & water temps and miles per gallon - but there's a lot more available if you want to play with it.