My own trip computer is usually optimisitc by 0.5 to 1.2 MPG, over the life of the car (about 15,000 miles).

I think that just about everybody in the very-high-mpg category does calculations by hand, using actual miles driven divided by gallons of gas.

Regarding prospective buyers coming into this site and having unreasonable expectations: I hope they would look at the average HCH MPG of around 46-48 MPG, understand the bias of most folks here who are acutely interested in getting better mileage, and set their own expectations somewhat lower (say 44-46). Then once they get the car, they will find that they develop some degree of more efficient driving skills, and get up to or above the average over time.