What price plummet? The only thing that has plummeted is the price of crude. Meanwhile the refining margins have zoomed.
In the last year crude oil has dropped about 25% from the high while gasoline has dropped only 10%. The oil industry doesn’t help their public image much when this much information is available.
http://gasbuddy.com/gb_retail_price_chart.aspx
Gas didn't climb as much as oil, why do you expect it to drop as much as oil?