One hand calculation that matches the DIC once does not disprove the 100s of hand calculations that are always shy of what the DIC represents. I track every fill-up on Fuelly, and every single one has been shy of the DICs readings. One or two hand calculations can be wrong, but not when I'm talking 20-30 fillups all being shy. I think most everyone else who has done multiple hand calculations of mileage has found them to be shy as well.I am a huge proponent of the in dash AVG MPG meter in the dash and have now proven that it is accurate. This weekend, I drove from a suburb of Akron, OH to Buffalo, NY and went through a full tank of gas on the round trip. I was surprised the car made the trip there and back on just one tank of gas. The mileage meter said 35.2mpg and I did the hand calculation which proved to be exactly the same. I was disappointed in the 35.2, but it was with the cruise set at 70mph and the transmission set in manual mode in 6th gear (auto trans car). This car continues to impress me each day. I actually prefer driving it now when compared to the CTS-V in the garage!
The DIC is, by and large, inaccurate and optimistic.