For the rest of this year, DON'T purchase ANY gasoline from the two biggest companies (which now are one), EXXON and MOBIL. If they are selling less gas, they will need to raise their prices to keep overall profits high. If other companies are selling more gas, they will be inclined to raise their prices. If EXXON and MOBIL also raise their prices, other companies will be even more inclined to do so (and vice versa).

The so-called "law" of supply and demand doesn't work all that well when it's in conflict with, say, economies of scale, or the power of virtual monopolies.