When you drive up to that gasoline pump next fall, and it says "25 cents", don't get your hopes up-it's not the deal you think it is.

The U.S. Metric Board has launched an investigation into the possibility of converting the nation's 1.5 million gasoline pumps over to the metric system. Such an action would make liters the new form of measure for gasoline, replacing the gallon.

Therefore the 25 cent price posted on the gas pump would be for a liter, not a gallon. And since there are almost four liters in a gallon, you would be paying almost a $1 gallon at that price.

That brings us to the reason the Metric Board has decided to act now.

With prices of gasoline nearing $1 a gallon, the oil industry is facing a $200 million expense to convert its existing gas pumps. Gas pumps now register prices up to only 99.9 cents. To take prices over $1, the machines have to be altered.

The Metric Board claims that if the industry would merely decide to switch now to metric measure-a move they see as inevitable in any case-it could save $150 million.

Board member Sydney Andrews explained that it costs nearly $200 to convert a gasoline pump to compute prices higher than $1 a gallon, while the cost of converting a pump to measure gasoline by the liter would be only $50.

At a meeting last week, the board ordered its staff to study the pros and cons of a possible conversion, and called for a public hearing on the proposal within 30 days. The Energy Department, major oil companies, pump manufacturers, gasoline retailers and consumers will be invited to the hearing.

Reaction in the industry to the plan has been mixed.

"I think it would be pretty good," said Roy Page of Cooper-Page Exxon in Arlington.

"It will solve some of our problems, particularly for dealers who own their own pumps and have to pay for conversion themselves."

But James Heizer of the Virginia Gasoline Dealers Association said, "Personally, I'm not too thrilled with the idea. It would be rather confusing to the public, which is an important consideration." Heizer said the majority of his group's members lease their service stations and equipment, and would not have to pay the conversion cost in any case.

A spokesman for the American Petroleum Institute, the oil industry's leading trade group, said the industry is divided on the issue.

"There is no consensus," said Gary Hirschel of the API. "Some companies are for it, some are against it, and some have probably not made up their minds."

Shell Oil Co. spokesman David Gross said his company is one of the undecideds." "Some of the pump companies have already told us that they have ordered retrofit devices that will allow their pumps to register prices over $1. We have been hoping gasoline prices would not go over $1. All we know for sure is we can't have two systems." CAPTION: Picture, Karen Stair, employe of Veede-Root Co., which makes gasoline pump computers in Altoona, Pa., shows pump converted to register over $1 a gallon. UPI