t's high time for the big oil companies to explain one of life's great mysteries; exactly how they fix the price of fuel at the filling station. The public has heard all sorts of explanations -- market forces, regional instability, refinery issues and so on -- but the logic behind the ups and downs (mostly ups) of gas prices is about as transparent as an IRS tax form. This topic is a burning concern not just because of costlier gasoline, which is, of course, a major drag on the U.S. economy and which affects all of our lives. It is also pertinent when it comes to diesel fuel, which, for no apparent reason, recently leapt up in price, well above the level of gasoline. Perversely, this has occurred just as consumer interest in fuel-efficient diesel vehicles is on the rise and as several major automakers are launching a new wave of advanced, clean diesel models.