Why do gas prices at the pump rise quickly and fall slowly?

When people notice that the pump prices rise, they top off the tank at the current price for that fuel already in the tank in the ground, which in turn makes the gas station owner order more gas. When it arrives and the price is more than the previous order, the price goes up reflecting the new delivery.
When people hear the gas prices are going down, they wait to fill because they want to benefit from the lower price. The gas station owner has to wait until the fuel in the tank in the ground goes down to a certain level before he orders another delivery at which time the pump price will reflect the price paid for that fuel.
In other words, it will take longer to empty the gas station tank when people think prices are dropping than it does when people know the price is going up.

Jeff in Sa-ra-so-ta!