It's really easy. Just make sure you switch the car type to hybrid or it will shut off every time the ice does.
I believe that I read in the manual that selecting the "ODO" setting on the eco score gauge shows instantaneous mileage. Don't have the manual in front of me....
That shows the mileage since switching the car into READY mode. http://www.toyota.com/t3Portal/document/om/OM52B58U/pdf/sec_2-2.pdf Page 187 of the manual, Page 9 of the PDF. While I'm at it, the 5 minute/Current consumption display being discussed earlier in this thread is described here: http://www.toyota.com/t3Portal/document/om/OM52B58U/pdf/sec_1-1.pdf Page 39 of the manual, Page 16 of the PDF.
I still say that if you want to watch "instant mileage" get an Ultra Gauge or spend more money for the better ScanGauge. You can watch many other things with these gauges, such as water temp, outside temp, and on and on. The new ScanGauge will give you the state of charge in the hybrid battery. By watching the intant mileage I can adjust my foot to get better average mileage. I like having the car computer gauge to see other things, like where I am in the power band. With an upcoming "hill" or over pass in sight, I can watch these two gauges to maintain my instant mileage and therefore keeping my average up. Just how I see it. 60 bux for the Ultra Gauge is a steal. Dan