Let me add that it would be easy to have the onboard computer keep track of fuel consumption, and warn the driver that the gas might be getting stale. It could issue a message such as:
"WARNING: Gasoline has not been refilled in 8 months. We recommend you drive the next 400 miles without recharging, to use up some of the old gasoline. Refill the tank when it is below 1/4 full." The Prius onboard computer monitors the gas tank, and resets the efficiency calculation when it is refilled. - Jed
