There is a thread on this at http://www.hihychat.com/forums/index...t=ST&f=4&t=255.

It appears that it is probably related to when your specific vehicle was manufactured. No one (as yet) has figured out how to enable it if it was delivered without the average MPG readout. I would think that a software update would do it, but Toyota would have to be convinced that there is sufficient need to warrant doing the work.

Good luck, and let us know if anyone has any success.