The only real value of "official" mileage estimates is to compare different vehicles. Consider the North American spec smart:
US Combined Mileage (2007 standard)..........42mpg
US Combined Mileage (2008 standard)..........36mpg
Canadian Combined Mileage (in US gallons)....44mpg
Identical car, three different numbers. Which is right? None of them. Each smart owner will get a different result, depending on their own usage patterns and driving style.
It's worth noting that the change from the EPA's 2007 to 2008 standards was especially unfavorable to small, fuel efficient cars and hybrids (most of which are made by non-US companies) and much less unfavorable to large, inefficient SUVs (most of which are made by US companies). Entirely coincidental, I'm sure...