The big plunge in oil seems to be finally having its effect at the gas pump. For the week ended today, the average price of a gallon of self-serve regular was $2.506 in the L.A. area, down from $2.556 last week and $2.620 the week before. All the cities surveyed by the federal Energy Information Administration saw declines, and in Cleveland, gas was averaging under $2 a gallon. Oil fell 86 cents on the NY Mercantile Exchange today and is now at $51.13 a barrel.