So I wonder, why the MPG discrepancy? With an opportunity to precisely measure the real MPG, why would Honda or Toyota or Ford have their calculations be off?

I understand that for some of you, the dashboard values are correct. But for a number of folks, myself included, it seems like the dashboard values are "optimistic". My difference seems to be about 1 MPG (e.g. 45 dashboard is really 44 measured).

I would not *think* that the automobile manufacturers would be trying to pull a fast one on us. I *might* think that the oil companies would pull something like this, but then I don't think they run the gas station equipment, do they? I think that the pumps themselves are supposed to be correct -- they are probably required to by law -- but then, I have heard that meters in taxis have been "miscalibrated" to the driver's advantage, so...

Anyway: does anybody have any theories why the mileage discrepancy might exist? Maybe a technical reason -- e.g. gas blends, evaporation, or something?