Ecodiesel: 19.46 MPG Towing 7,020 lbs.

The engineer stated the computer needs to know exactly how much fuel it is injecting into the engine at all times in order to meet emissions so it is much more accurate than hand calculating.

Either way is alright for general use, however if your are testing, seems to me you would start with an empty, tank, fill with known quantity, drain and measure remaining fuel at end of run.

I respect engineers but don't forget that the cold wasn't going to affect the seals on the Challenger.