My wife discovered this website, which has a number of links with technical information on the Prius, particularly relating to its regenerative braking and fuel efficiency. One of the links takes you to a detailed chart of fuel economy at different temperatures and speeds. Unfortunately, the site does not describe the distance used to conduct the test, but the rigor contained throughout the site tells me that it was probably done over a significant distance to create a valid result.
http://os2.mystarband.net/