"why do price reductions always take months to show up at the pump, but increases in oil barrel prices send GAS (already refined, and at the pump) up immediately?"
Retail price is generally on a fairly fixed margin over wholesale.
With this in mind:
Because if there is going to be a supply crimp increase (e.g., a war or hurricane) people all rush out and fill up their tanks w/ cheap gas. Thus, the "cheap" retail gas is rapidly replaced with more expensive wholesale gas.
If prices are falling, people drive until empty, as it's cheaper later. Thus, the expensive retail gas is slowly replaced with cheaper wholesale gas.
Basic consumer dynamics.
hmmm good point- I always make the mistake of thinking a static economic picture
thanks