It is Gal per hour. It gets this reading from the OBD2 port so the computer is calculating this somehow. As Vic said it is a good reference tool to determine the efficient operating spots for this engine. The reason mine doesnt work is not because of the fuel side but because it displays 0 MPH about half the time so my average always gets screwed up. Maybe I got a bad scangage or my OBD2 port has a weak signal? Overall it was not a lot of money and I like the ability to monitor my coolant temp in the winter on it. 50C on the scan gage is where my dash gauge starts to move. A lot of people on here will tell you wait for the gauge to move as a warm up indicator. The are wasting time and fuel on that because 50C is plenty warm befor starting to dirve. Check out a cost of a OBD2 code reader, a very handy tool and this does that too, just download the codes off this site and take put them in the truck and you will always know what is wrong with your truck as soon as it happens.