Based on the fact your vehicle’s calculated MPG is realistic, it sounds like the fuel flow sensor in your vehicle is one of the more accurate ones -- I'd guess it's only off by 5-10%. So if it’s measuring 19 MPG, 5-10% equates to 1-2 MPG, meaning your true MPG is probably more like 17-18 MPG. Only way to get an estimate of the error is to hand calculate your MPG using the pump method. The accuracy of the pump method will depend on how much is in the tank when full, so ideally the gallons of gas in the full tank should be the same. The best way to do that is to use the same gas station fuel pump for both tanks -- and make sure the station fuel pump has been calibrated recently (should be a sticker on the pump). Fill the tank until the fuel pump cuts off, wait 30 seconds, then squeeze the pump handle again until it cuts off a second time, then stop. Before you start the car and drive off, reset your mileage trip meter and the MPG calculator in your car. Drive the car as normal until the fuel gauge is close to empty, the go to the same pump at the same gas station and fill the tank up using the same procedure described above. Before you leave, write down how much gas in gallons it took to fill the car up, the miles you traveled, and the car's MPG computer estimate of your MPG. When you get home, divide the miles you traveled, by the gallons it took to fill the tank, and compare that number to what the car MPG computer calculated. Do this for 5 fill ups and you'll get a sense of the accuracy of the MPG computer in your QX60. Perhaps you’ll discover that your vehicle is the unicorn, and your MPG computer is accurate!