"Most oil companies follow the practice of grouping their retail outlets into geographic or market zones and charging their branded dealers in different zones different prices for the same brand and grade of gasoline" from the same source, the Senate report says.

"Each oil company has its own zone system. The number of outlets in a zone, the shape of a zone and the number of zones in a particular area vary from zone to zone and company to company. In recent years, zone size has been shrinking; some zones now contain only one outlet," the report says.

It notes, "Station dealers argue that the zone pricing policy is unfair, because it allows an oil company to charge gas stations in nearby geographical areas — sometimes on the same corner — different prices for the same gasoline."

It says zone pricing is designed in ways to "enable particular stations to be able to charge higher prices without losing too much volume to nearby competitors." It said consultants who design zones "believe they can determine how much prices can be raised at a particular station before consumers will drive to other nearby stations."

Gas station owners say a related practice is "redlining." A fact sheet from the Western Petroleum Marketers and Retailers Association says that is "the practice by refiners to prohibit jobbers (independent gas station chains or distributors) from selling the refiner's branded products in a certain area. In some instances, refiners restrict growth."

It protects an oil company's own brand of stations. But the gas station owners' association says, "Redlining and zone pricing hurt consumers by reducing competition through elimination or restriction of competitors."

Hill, with the Utah Petroleum Marketers and Retailers Association, said the practices have caused problems in Utah on occasion.

"For example, it was very noticeable a few years ago in the Rose Park area. An oil company had a set price on one side of the street where it was 'subsidizing' a station (that sold its brand), and on the other side of the street it was charging much more. Some small retailers were affected."

Peacock, however, said most differences in prices to local gas stations are based on the volume of what they buy. "It's like when Wal-Mart purchases in bulk and quantity so it gets a discount. . . . The same is true for large (gas) wholesalers or jobbers. They are able to buy in quantities that allow them to get a little better price."

Of note, when a motorist buys gasoline of a certain brand, it does not mean it was produced by that company's refineries. About one-third of stations in the country are "unbranded" and may sell gasoline of any brand.