After reading this piece in the Daily Telegraph claiming prices at the pump were not falling as fast as the price of crude, I decided to see what was going on. I went to the Energy Information Administration website, from which I downloaded the weekly retail gas prices and the spot crude oil prices and compared them to each other. They have records for the latter going back to the beginning of 1986, but for the former only to mid-1990 (with a mysterious gap of 6 weeks between at December 3, 1990 and January 21, 1991). I then charted the price of gas (Weekly U.S. Regular Conventional Retail Gasoline) as a percentage of the price of oil (Weekly Cushing, OK WTI Spot Price FOB):

Gas-Oil
While it is true that the over the last few weeks crude prices have dropped faster than gasoline, gas prices as a percentage of the cost of crude are near the lowest they have been for the dates for which data are available. And when you compare the relative prices:

Gas-Oil2
You can see that historically the prices have been pretty constant, but that crude ran up much more than gasoline, so the price at the pump was actually less than what it would have been had gas moved in lock-step with crude prices.