I have heard just the opposite that the EPA ratings were a bit optimistic.
I also think they are a bit optimistic for most, Toyota being one of the exceptions.
EPA ratings are in fact established by the manufacturer itself, using of course, idealistic conditions.
EPA just randomly check some of them (hum ! hum ! Ford, Kia, Hyundai, would you come in the office for a moment?
).
Corollas numbers have been double checked.
Actually, my Eco Base get around 50 mpg at 70 mph stable speed on a highway.
For a LE (rated at 38 mpg hyghway EPA) Consumers Report got 43 mpg on the highway.
So Toyota is playing it a bit too safe.