Soon, "pay at the pump" may become "pay before you pump" -- months before ever filling up.

Two U.S. companies are looking to offer a pre-paid card that stores gallons of gas, rather than dollars.

The cost of these pre-paid gallons will depend on the price of gasoline at the time of purchase. In effect, customers can lock in a price at some point in time for a certain number of gallons to be used at any time within a year, said Marge McDonnell, director of credit card services for Gulf Oil, one of the companies planning the offer.

McDonnell added that Gulf Oil, a Massachusetts-based company with 1,800 Gulf brand gas stations nationwide, hopes to start the initiative next year.

"The details are still being formulated," she said. "I think that people are receptive to it. The program will be a good fit for people on certain types of budgets."

Fuel Bank, a subsidiary of Premier Points Inc., is another company intending to sell gallons of gasoline for a pre-paid price.

"We're in planning stages also," said Rod Senior, CEO of the company.

Fuel Bank, which is not an oil company, plans to sell "virtual" gallons of fuel on the Internet and issue a card sometime early next year so that customers can fill up with the already purchased gallons at their local gas station.

The gallons on the card are price-protected for up to a year, Senior said.

He said he thought of the idea after engaging in a similar business venture in Europe.

He explained that he bought, stored and sold large amounts of diesel fuel. Selling "virtual" fuel presents fewer logistical problems, he said, but some more possible obstacles.

For instance, he said, there are differences in gas taxes from state to state that affect the price consumers pay for a gallon.

Fuel Bank is in the process of working out obstacles like this, he said.

Senior said the time is ripe for this sort of economic arrangement because the Internet makes it easy to sell "virtual" gallons in real time and a futures market, which speculates a future price and buys "shares" of the commodity on world markets, allows for investment of pre-paid funds.

If the price of gas were to skyrocket, the companies involved would still be able to turn a profit because it will have invested, or hedged, the gallons into oil futures or other investments where it will make a profit, McDonnell said.

If the price of gas decreased for any reason, customers could choose not to use the card, she said.

Because the fixed-price gallons will be available on the card for a year, the best time for customers to use their gallons will be when the price of gas is higher than what they bought it for.

Economics professor Mark McLeod said pre-paying for gallons of gas is an interesting idea.

"College tuition offers the same thing in a lot of states," McLeod said about the concept of prepaying for a product in the hopes it will make for a good deal in the future. Parents can buy what amounts to "tuition futures" when their children are born. They pay the money up front in the hopes that it will save them money 18 years down the road, McLeod said.

"It's such a politically driven system, too," McLeod said of the oil market, which is sometimes affected by wars and policies of foreign countries.

McLeod added that the plans of Gulf Oil and Fuel Bank sound like a way to reduce uncertainty in consumers.