The Department of Energy agreed yesterday to consider the environmental impact of its proposal to alter petroleum pricing regulations, an action that will delay an expected 2.6-centper-gallon increase in the price of unleaded gasoline until at least February.

The agency already has drafted an impact statement claiming that the overall deregulation of gasoline prices will not have a "significant" environmental impact. But DOE concedes that there will be "some adverse environmental impact" from increased car exhaust emissions.

The extra auto exhaust emissions would result from motorists switching from use of unleaded gasoline to leaded gasoline as the price differential increases between the two, according to the impact statement. DDE's Economic Regulatory Administration said there is now an average price difference between the two gasolines of 4.4 cents and that the figure would rise to an average of 7 cents if the change in the regulations takes effect.

The controversial changes in the pricing regulations would allow oil refiners to pass through to consumers additional non-gasoline-related costs.

Two consumer groups, Public Citizen and the Center for Auto Safety, and Ralph Nader sued DOE to block the new regulations, which originally were slated to take effect Dec. 1, U.S. District Court Judge Aubrey E. Robinson Jr. dismissed the suit yesterday as moot when DOE agreed to study the environmental impact of the changes.

Nader and the consumer groups have contended that as the price differential increases between leaded and unleaded thousands of motorists are switching to the dirtier leaded gasoline, even if their cars are built to hanndle unleaded gas. This will increase air pollution, Nader and the consumer groups claim.