If you are the kind of motorist who likes to save money by paying cash for gasoline but who doesn't like pulling away from the pump with an empty wallet, some oil companies have a deal for you.

It's called the debit card and, like a credit card, it allows you to ride the plastic. But unlike a credit card, the ride is very short -- instead of paying when the bill comes at the end of the month, the money is "debited" out of your bank account right away.

The oil company gives you the cash price because it doesn't incur the cost of extending credit to you, and you don't have to have a wad of greenbacks to fill 'er up at the lower price.

A deal you can't refuse, right? Well, maybe, but consumers who don't check with their banks first could be in for a nasty surprise. Some banks charge fees for debit services, some as high as 50 cents a transaction.

So if you buy 10 gallons and use your debit card to save 4 cents a gallon, a 50-cent transaction fee more than eliminates any economic benefit. Under those circumstances, a consumer might well prefer to use a credit card and at least have the benefit of the "float," or lag between when the fuel is purchased and when the payment is due.

Mobil Corp. and Exxon Corp., which have sharply expanded their debit card programs this year, say they don't have figures on how many banks are charging fees. Both, though, say they try to persuade financial institutions not to.

"We strongly encourage them not to charge, but we can't forbid them to," said Carole Edwards, spokesman for Mobil's U.S. Marketing and Refining Division in Fairfax. "We have been more successful with some banks than others."

"There are no charges on our part, and no fee at the station," said Les Rogers of Exxon Co. USA in Houston. "If a fee is charged by the bank, we have no control over that. We had hoped customers would not incur one, but if a bank does impose one, that's beyond our control.

"It's my impression that {fees by banks are} a relatively rare occurrence," he added. "We encourage our customers to check with their financial institution to determine its practice."

David Robertson, vice president of the Nilson Report, a Santa Monica, Calif., newsletter that tracks the credit card industry, noted that the debit card industry is "still in its infancy" with many issues yet to be resolved. Other retailers, as well as oil companies, are experimenting with debiting, and "there is a philosophical battle that's being drawn" between the financial institutions and the retailers "over who's going to pick up the tab for these services."

In addition, he said, "there is further debate within the financial industry itself -- what, if any, fees {banks} should pass on to consumers if {they} are really interested in getting them to use these cards for multiple purposes."

Currently, debit card offerings and fees vary greatly. Mobil, for instance, offers customers a choice. Holders of Most or Cash Flow automated teller machine cards may use them at Mobil stations, or they may apply for the Mobil+ card, which carries an annual fee and which may be used either as a debit card or as a conventional credit card.

Exxon offers the debit feature as an option on its conventional credit card, for which there is no fee. In addition, Cash Flow customers may use that card at participating Exxon stations. Cash Flow is owned by Sovran Financial Corp. and is available only to customers of Sovran-affiliated banks. Thomas H. Lewis of Cash Flow said that Sovran does not charge customers for using their cards this way, nor does it charge bank customers who use Mobil or Exxon debit cards.

Arco, which markets gasoline in five western states, has no card of its own but offers debit sales to holders of a variety of ATM network cards. Unlike the others, Arco itself charges a 10-cent transaction fee. It says banks may add a fee of their own if they wish to, and a small number do, averaging about 25 cents.

Different banks treat the cards in different ways, based on both costs and marketing considerations. Some process debit transactions without charge to the customer. Others apply their minimum balance requirement: If the customer maintains a certain minimum balance, the transactions are free; if he lets his balance fall below the minimum, fees kick in. Others treat the cards as "foreign" ATM cards -- those outside the bank's own network but to which its customers have access -- and charge a flat fee.

For most banks, "I imagine it will be a marketing issue," said William Petrarca, senior vice president of National City Bank in Cleveland and chairman of an American Bankers Association committee that follows electronic fund transfer issues. But he noted that the Mobil and Exxon debit cards are "such a new concept" that banks may be worrying about encountering some unexpected costs from them and pricing to protect themselves.

In addition, there is an important difference between the Mobil+ and Exxon cards and other debit cards, making the oil company cards more work for the customer's bank.

The Nilson Report's Robertson said that ATM network cards, which are seeing growing use in "point-of-sale" debiting at other types of retailers, involve "financial institutions talking to financial institutions." With the Mobil and Exxon cards, though, transactions go from the station to the oil company's computers and then through the Automated Clearing House, which handles interbank electronic transactions, and on to the customer's bank. The process usually takes about two days.

"These are really electronic checks," Robertson said. Under the Federal Reserve's Regulation E, he said, the customer's bank is responsible for posting the transactions correctly and keeping records of them.

Said Petrarca: "Banks pretty much know what 'foreign' ATMs cost, but because {the Mobil and Exxon approach} is so new it's difficult to tell what the costs are. There are going to be some overhead costs" with problem transactions, such as debits posted to the wrong accounts, so "it's premature to know" what the cost is going to be.

Mobil's Edwards said the company realizes that the banks face uncertainties. "From the bank's point of view, it's tough to justify {accepting the cards} without charging a fee," she said. Indeed, "the major problem in the debit card system is getting cooperation by banks. The pluses for us are immediate, ... but they are not immediate for banks. It can take years to prove it was a good thing for them to get into."