I've calculated the long-term average by tracking how much fuel I put in the tank and how much the odometer goes up, over many fillups. From that I get about 55mpg.

The efficiency in specific situations is from the on board computer.

How do you calculate it?

> In the real world, I haven't seen hybrid drivers that actually get what's advertised

Come over to priuschat.com sometime and you'll find plenty of them that get the advertised numbers and better.

EDIT: Or check out fuelly, which calculates efficiency using the same method and you'll find plenty of people who get better than the official numbers: http://www.fuelly.com/car/toyota/prius