[Diesel_Dave]

Bottom line is that no system is completely fair.

There are pros and cons to every conceivable way. The whole "% over EPA" metric is designed to represent a "% over baseline". If everybody's real-world duty cycle was exactly the same as the EPA certification test, then we'd be all set. Unfortunately, that's not the case.

We have some people who run:
mostly highway
mostly city
in very warm climates
in cold climates
mostly short trips
mostly long trips
in mostly heavy traffic
in mostly light traffic
over very hilly terrain
over flat terrain
etc.

Not to mention, not all vehicles have EPA ratings (European vehicles, etc.)--even in the USA. My truck, for example, doesn't have an EPA rating because it's in too high of a weight class.

Not to mention that different people want to compare different things. For example, hypermiling skill, vehicle modifications, money saved, fuel saved, CO2 emissions, etc. Depending on what you're trying to compare, different metrics may make sense.

So, nothing is perfect, but IMO, % over EPA is probably about as good as we can hope for.