It updates every 10 seconds or so...how often do you need it to update? With the '06+ Prii the lifetime average mpg does not reset with each fill up. So, the more miles you have the less change you'll see in the average. If you hit the reset button at each fill up you'll see a tank average MPG displayed. Even then, toward the end of a tank, the average won't change very much except for the most extreme conditions/variances from the average. IOW, if your tank average at 400 miles is 55mpg, but then you begin a long high speed uphill mountain climb (est. mpg for climb alone = 30mpg) you'll be able to see the average mpg drop slowly during the climb.
