The price of gas is set to make maximum profits. Thats the law of supply and demand. To truly understand this principle, you would need to take some courses at a local college, it would be difficult to explain in a single post here, but here goes anyway.
You could set the price of gas at say $1M, but you would be lucky to sell one gallon at that price. To operate a refinery to produce just 1 gallon of gas, it would probably cost you more than $1M, so you would loose money.
You could set the price at $0.01/gallon and you would sell every gallon that you could produce, but it wouldn’t cover your costs.
At any price point, a specific amount of gas will be sold. The cost of producing that gas is determined partly by the quantity of that demand. Two independent curves are developed. One curve is the price demand curve, as price goes down, demand increases.
The other curve is cost production curve. Generally as production goes up, cost goes down, but there are bumps in that curve. For example, lets say that you have 5 refineries each producing 10 million gallons per shift. With three shifts per day, you have a total capacity of 150 million gallons per day. There will be a bump in the cost production curve at each 10 million gallon point because of having to put on another shift. There would be a huge bump between 150 million and 150,000,001 gallons because you would have to start another refinery.
Now the two curves are plotted on a single graph. Where the price demand curve is above the cost production curve, that is profit. The point where there is the greatest separation between the two is the point of maximum profit.
Taxes are part of the cost production curve. A change in taxes will move the maximum profit point up or down on the graph. Most of the time, the maximum profit will move in the same direction as the taxes, but not necessarily the same amount. Near the bumps, a change in the tax could have the opposite effect on the price. Thats hard for people to accept, but it does.
The oil companies have computers programmed to determine this maximum profit point at least twice a day for every gas station they have. That is why you can see different prices for gas stations from the same company only blocks apart. Each oil company has their own cost production curves because of their different overheads, so the bumps don’t occur in the same places.
An $0.18 drop in the tax may not translate to an instant $0.18 price drop. It could drop more or less than that amount, and under just the right circumstances, it could even cause the price to rise. There are so many other factors involved in these calculations that it would be impossible for us to determine the exact affect the change in tax actually has on the price of gas. I.e, the price of oil goes up that day so we accuse the oil company of keeping the tax savings for themselves.