Hello,
Since I received my new Fit 09, I have kept a log of the computer mpg and the calculated mpg.

Method:
I would fill the tank until full. To know it is full, I waited until the gas pump overfill security stopped the pump. I then would shake the car side to side, then pump again twice until the pump stops again. Not really scientific since I never used the same pump, but this is the best I could achieve.
I have excluded from the data the first fill (I think I got somethings like 33 mpg calculated).
My driving is mostly state road driving in the flat Florida @ 55-60 mi/h (south and Central and the Keys).
I am still driving with the original break in oil from the factory (~10,000 miles and odometer says 20% of oil life, will switch to synthetic this week, see thread here).
I rarely shift gear above 3500 rpm and I always look ahead for traffic light, even putting at neutral at times to avoid stopping.
My tires are inflated at 36 psi (will inflate to 40 psi soon).

Results:
The model is: [computer MPG]= 1.16 [calculated MPG]
The ratio is 1.16 +/- SD 0.07, Min= 1.04, Max= 1.35, median= 1.15

Computer MPG = 43.4 +/- S.D. 2.35, Min= 40.40, Max= 48.70, median= 42.80
Calculated MPG= 37.3 +/- S.D. 2.7, Min= 33.54, Max=41.13, median= 36.94


Conclusions:
Overall, there is a good agreement between the calculated mpg and the computer mpg. However, the computer consistently over estimates the mpg.
Unlike others who sometimes are way higher than the EPA MPG for the Fit (I have seen 47 mpg!!!), I am well within the 35 MPG on the highway.
I do not know how these people achieve this, as I believe that my driving is really economic and maybe sometimes a bit detrimental to security and to the engine.
I will continue to monitor my mpg after the switch to full synthetic (Mobil one most likely since Amsoil is quite expensive, and good =not FRAM, filter). I will also inflate the tires at 40 psi

Stay tuned for more in 6 months or so.

Hope this helps.
Hydro