Gasoline is essentially a generic product. The only thing that distinguishes brands is the fraction of a percent of proprietary additives dropped into tanker trucks before delivery to retail gas stations. So why can the price per gallon differ by as much as 50 cents at stations only blocks apart?

The answer can help you save hundreds of dollars per year. For example, on East Washington Street in Indianapolis last July, you could have paid $3.07 per gallon for regular at a Speedway station or driven 2½ blocks to a Clark station and paid only $2.56. That’s a $12.75 savings on a 25-gallon fill-up.

There can be wide price variations within the same brand, too. In New York’s Jamaica, Queens, neighborhood, one Sunoco station was charging $2.93 a gallon for regular in July, while another Sunoco a mile away was charging $3.30.

The differences trace back to who owns the pumps. Gas stations that sell a specific brand of gas may be owned and operated by the oil company and leased to an independent dealer, or owned by a “jobber” who contractually commits to buy branded gas from that company. Independent stations can buy wherever they can get the lowest spot price.

The underlying costs for each business arrangement can dramatically affect the price at the pump. Prices can vary within the same brand because of zone pricing, a practice in which oil companies adjust their wholesale prices to the prevailing rates in different parts of town to keep their stations competitive and profitable. Contrary to popular belief, independents don’t always have the best deal.

When oil prices are relatively low or falling and supplies are adequate, independent gas stations tend to have the lowest prices because they’re free to shop around, says Michael Burdette, a senior analyst at the federal Energy Information Administration. Super-size independents or hypermarketers, such as Costco, can afford to market gas for little profit; convenience-store gas independents make more profits selling coffee and cigarettes than gasoline, Burdette says.

When oil prices are relatively high or rising and supplies are tight, brand-name gas stations will tend to have a pricing edge. That’s because the brand-name companies make sure their own stations get scarce supplies first, at contract prices that insulate them somewhat from hikes, Burdette says. Scarcity pushes up spot market prices and independents’ costs.

Shop using the Internet. To find the best deals in your area, check prices at California AAA site, as well as at www.gasbuddy.com, www.automotive.com/gas-prices, and www.gaspricewatch.com.

Break the same-station habit. Instead of always filling up at a familiar gas station, monitor prices all along your normal travel routes and buy where they’re lowest.

Avoid airport gas-price traps. Rental-car agencies’ overcharging to refill the gas tank on returns tends to push up prices at gas stations near airport car rentals. Don’t wait until the last minute to gas up.

Buy out of state. If you live near a state with lower gas prices, fill up there.