I contend that the onboard FE display is going to be much more accurate from a tank to tank basis than hand calculation because the data source and the way the data is compiled for the vehicle's calculation of MPG is much more accurate than the source data available to us for calculation.

The car calculates MPG based upon fuel metered to the engine over distance traveled. We calculate FE based upon tank fill up over distance traveled. We and the car see the same distance traveled, however the onboard computer knows how much fuel was metered into the ICE. The only thing we know is how much fuel we were able to put in the tank. Bladder or not, that tank fill can vary from filling to filling. A few ounces one way or the another can have a huge impact on FE on vehicles which get exceptional economy. On the other hand, the metered fuel is a known value which is stored with the onboard computer system, compared to the known distance traveled and the vehicle comes up with an accurate average based on values that are fully known with no "fill-up variance" error.

Now, if all gas pumps shut off at the same place and time on a vehicle, or we had a trans parent filler indicator on the side of the vehicle so we could see the actual fuel level in the tank, then we could refill the tank to the same point each time we refilled and do a calculation. I would suspect that if we could do that, we would get an FE number that was quite similar to what the vehicle calculated.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)