Funny, Congress is 'posed to have some hearings on the gas issue soon. I can't remember where I read/heard this, but there has been talk of breaking up the major oil companies that merged in the 90's. Congress is "worried" that oil companies are price gouging. Gee, you think?? How long has it taken to figure this out...your government dollars at work.
However, in America, with only three major oil companies, there technically is no monopoly on gas, but the three companies do control distribution and pricing, so naturally they will all be in league with each other on prices. Where each company jumps ahead of the others is within the company itself and who can cut costs. Exxon-Mobil gets the nod here because they control most of their oil from the time it is loaded onto a ship until it is put into your gas tank. Self contained organizations don't have to pay the middle man and are able to cut overhead. So actually there is a monopoly with only three big companies in the business and they buy out or push out anyone who stands in there way.
I know that I'm getting a tad into conspiracy theories here. I've also talked at length with my dad who is a petroleum engineer and worked for Exxon and Shell in the 70's and early 80's before he started his own production company. Much of the info he's given me has been regurgitated here and in several other posts on this subject.