A congressional watchdog subcommittee has challenged the credibility of the Department of Energy, charging that agency with withholding data that would have shown the possibility of dangerous gasoline shortages within three years if gas were decontrolled.

DOE has tentatively proposed gasoline decontrol, but the House subcommittee on Oversight and Investigations said, in a report issued yesterday, that DOE's "case for gasoline decontrol is unlearned, distingenuous, and unpersuasive."

A DOE official said the study in question was only a preliminary report that was discarded because of questions raised about the accuracy of its assumptions.

"The primary issue raised in this report," the study states, "is not whether we should deregulate gasoline prices . . . but on the competence and credibility of the DOE in addressing the issues surrounding gasoline decrontrol."

The subcommittee cited a report written by a DOE official and previously published in the Washington Post that indicated the possibility of "critical" shortages of as much as 400,000 or 500,000 barrels of gasoline a day if gasoline prices were decontrolled by 1980.

During the height of the oil embargo five years ago, shortages ran only as high as 280,000 barrels a day.

"The failure of DOE to discuss this potentially grave problem in their published assessments of the impact of decontrol raises serious questions as to the credibility of the department," said Subcommittee Chairman John Moss (D-Calif.) in a cover letter on the report.

The previous withheld DOE studies projected that under decontrol gasoline prices could rise by an estimated 20 to 40 cents per gallon, Moss said.

He called on DOE to perform "complete detailed studies on the environmental, supply shortage, and price competitive implication of gasoline decontrol."

The report also addressed the possibility that if gasoline is deregulated at the pump, the price difference between leaded and unleaded gasoline will soar.

Environmentalists and consumer groups have claimed that many consumers already are switching from unleaded gasoline to leaded because of large price advantages of doing so - despite the fact that all new cars have a device that is only supposed to allow unleaded pump nozzles into the car's gas tank.

Motorists are apparently removing the restrainers and pumping leaded gasoline into their cars, in an effort to save up to eight or nine cents per gallon. That spread will get worse if gasoline prices are deregulated, consumer groups contend.

Environmentalists claim that wholesale switching would occur if the spread becomes wider and the result would be considerably dirtier air, since leaded gasoline is a much heavier pollutant.

"DOE had no basis for concluding that gasoline decontrol will not have an adverse environmental impact," the Moss report stated. "If fact, the evidence strongly suggests the contrary."

The Environmental Protection Agency has said that unleaded regular gasoline is averaging 4 cents a gallon higher than leaded, but the spread jumps to as much as 15 cents in some areas.

There are penalties for switching illegally, but only against the gas station owner or attendant, not the motorist. With the switch to self-service gas stations, gas station employes often have little control, over the actions of customers.