HINT - since both you and the DIC use the same MILES-number at each tank-fillup to calculate MPG, all you really need to compare are the two GALLONS-used values: (1) the GALS calculated by DIC (ususally 5-10% optimistic) and (2) the GALS that you actually pumped into the tank(*).
Tracking & comparing these two numbers gives you an excellent idea of how much "off" the DIC-computed value is...and, seemingly, that error is lowest with manul ECO models and highest with automatic non-ECO models.
(*) - eventually, the "Law of Large Numbers" will overcome and "smooth" your slight ± (over/under) tank-fillings, especially if you use the "one-click" fillup method.