The Energy Information Administration keeps track of the total amount of gasoline in storage in the United States. This information is of interest because historically when the amount is low, the price of gasoline rises. This can happen regardless of the price of oil, the bottom of the pyramid in the gasoline production chain. Short term gas prices are set by the relation between demand for gasoline and the supply available. The EIA publishes a report called the Weekly Petroleum Status Report where you can always find the latest updated information.
In the graph, taken from the current Weekly Petroleum Status Report you can see how in September 2008 the amount of gasoline was well below the gray average band. We also had record fuel prices in that time. Right now the national gasoline supply is above average. Good news for drivers! Keeping an eye on this graph is like keeping an eye on the nation's fuel gauge. When it starts dropping below the average range, watch out for high gas prices!