Some Washington area contractors have begun to impose special surcharges onto customer bills to offset the rising cost for the gasoline they use in the trucks carrying men and machines to job sites.

The surcharges typically add $2 to $5 to the price that customers pay for such services as the repair of leaky faucets, cooling systems or water heaters.

Contractors who have adopted the new pricing system defended their actions yesterday, saying that fuel surcharges are easier to explain to customers than increases in minimum rates.

"If we have a fee that breaks out the costs for fuel and maintenance, the customer understands," said Bill Shreve, a Falls Church plumbing contractor.

Some customers, however, haven't understood.

Fifteen to 20 of Shreve's customers, for example, refused to pay the surcharges during the past year. In April, Shreve raised his fee for truck fuel and maintenance from $3 to $5.

Local government consumer officials said yesterday that fuel surcharges -- also known as delivery charges and cartage fees -- are legal so long as they are disclosed in advance by the company.

"A contractor can use my name he wants for any charge he passes on to consumers, and it would be legal," said Fred Goldberg, general counsel for the District of Columbia Office of Consumer Protection.

"But if it is sprung on the consumer after prices have been agreed on then it isn't legal and the consumer shouldn't pay it," Goldberg said.

Other consumer leaders expressed concern that contractors are using the surcharge system to slip through rate increases.

"It could be a ploy to raise rates," said Ron Mallard, director of the Fairfax County Department of Consumer Affairs. "People are almost numb from fuel increases and the contractors may think they can slide through their increases with the (public) utilities and others now imposing fuel surcharges."

Mallard also said that the use of a fuel surcharge by a service contractor can distort prices and make comparison shopping more difficult for consumers.

Fuel surcharges are most commonly associated with electricity and gas companies, which have been using the system for years to recover fuel costs.

But the idea has spread in recent months. The Interstate Commerce Commission approved a fuel surcharge last summer for long distance truckers.

Earlier this month, George Washington University imposed what may be the nation's first campus energy surcharge -- a $50 a semester fee starting this fall from each full-time undergraduate to help pay fuel bills in the 1980-81 academic year.

The surcharge practice began to blossom in the contracting business this spring, with some businesses imposing the fee for the first time and others increasing existing charges.

Contractors blamed spiraling gasoline costs as well as new delivery charges by their wholesale suppliers.

"I didn't want to do this," said C. W. Fields, an Arlington contractor who does plumbing and gas fitting work. "But I have six trucks on the street at all times getting 8 to 10 miles per gallon and I have to pay $1.20 a gallon for fuel. I have to take it out of somewhere."

Fields opted to take it with a $2 fuel surcharge. Since April, the statements mailed out to his customers have listed a price for material, labor and fuel surcharge.

In addition to the gasoline cost increases, Fields said, his wholesale supplier now charges him a delivery fee for each truckload of materials needed for a job.

The typical delivery fee is $5 for each shipment, according to Washington area wholesalers.

John Gullett, a spokesman for the Noland Co., said his firm imposed its $5 fee on Feb. 1 "in an attempt to isolate the problem area of fuel and other maintenance cost increases." Noland is a wholesale company handling plumbing, heating, electrical, refrigeration and air-conditioning supplies.

The company, which never had a delivery charge in its 65-year history, believed the fuel fee to be the most equitable way to recover costs, Gullett said. An across-the-board increase in prices, he said, would have raised the cost for customers who buy from Noland over-the-counter outlets.

On industry leader questioned the surcharge practice.

"I wouldn't be in favor of it," said Jack Strother, executive director of the National Association of Plumbing-Heating-Cooling Contractors. "It's difficult to estimate fuel costs -- and who is to say what the surcharges should be?"