That... as far as I can tell... has MUCH more to do with the inaccuracy of the shutoff on the pump than any accuracy/inaccuracy of the NAV or your calculations. I have a ScanGuage. They tell you to calibrate it by pumping the tank full, resetting it, driving until the gas is pretty close to empty (1/4 tank or so) and then you check what the Scanguage says and what the pump says. You use the same pump for both fillups. You enter the % difference into the scanguage. Now I've done this NUMEROUS times at the same pump, on one clickoff of the pump. My % has ranged from -5% to nearly +10%. The accuracy one gets from the pump number just is NOT very good. They don't have some fancy shutoff mechanism, they just use back pressure and its not terribly accruate... doesn't need to be... it just keeps you from overfilling the tank. The amount of gas dispensed is pretty accurate but there is little guarantee the tank is exactly as full as it was the last time.

After alot of fiddling with the scanguage I've pretty much just looked for the most common reading and set the scanguage adjustment to +6%. and what do you know...my NAV usually agress with my scanguage to better than 1 MPG until its clear that the NAV has run past the end of its memory.

In short... I think the NAV is "fairly accurate" at least for my FEH and my driving style so I wouldn't put much stock into the differences between it and hand calculations--they just aren't very accurate until you average a BUNCH of them together over several tanks.

I will say.. that at around 2000-3000 miles I definitely noticed a bump up in my mileage, but I also bought mine in the winter.