I like the flat road test idea. But, if there is a problem, could you tell whether it was in the odo or speedo (without further testing)? Maybe you could ask a cop-in-wait to test your speed for you, then circle around for a run by him. But surely there is a more convenient way.
As to the question of Honda trying to sneak 2% past its consumers, I can hardly imagine that it was an honest mistake. (I like my 03 CR-V, but have wondered since it was brand new about that odometer, and just never bothered to measure it because it didn't seem off enough to bother. I haven't yet received anything from Honda about it, either.) They have some of the best auto engineers in the world, the best available technology at their disposal, and a reputation for high mpg to maintain. In the long run, it's not THAT big a deal, as their cars still serve their intended purpose exceedingly well, but it is rather disappointing.