The Carter administration is contemplating a federal tax that could [WORD ILLEGIBLE] as much as $500 to the sticker [WORD ILLEGIBLE] of new gaz-guzzling cars, according to the administration and congressional sources.

The administration also is considering [WORD ILLEGIBLE] of up to $500 to those [LINE ILLEGIBLE] cars that get high mileage[LINE ILLEGIBLE]

[WORDS ILLEGIBLE] efficiency tax, based on a [WORDS ILLEGIBLE] standard, is one of a [WORDS ILLEGIBLE] under consideration [WORDS ILLEGIBLE] lead to reducing an- [WORDS ILLEGIBLE] consumption.

President Carter has said he will announce his energy program April 20.

Another tax measure under consideration is aimed at natural gas used as boiler fuel by utilities and other industries. The administration has made no final decisions, sources said.

Because of price controls, which have held natural gas prices below those for other fuels, and clean air requirements, many industries have been reluctant to convert from natural gas to coal.

A tax on industrial use of natural gas would raise the price of the gas to the equivalent price of imported oil - now more than $12 a barrel. Such a tax would be directed at what the President's energy adviser. James R. Schlesinger Jr., calls "ending interfuel competition."

Schlesinger and Federal Energy Administrator John F. O'Leary have said repeatedly in recent weeks that Carter's energy message would stress increased conservation and greater dependence on coal.

The 1975 Energy Policy and Conservation Act set mileage efficiency standards to achieve an average of of 27.5 miles per gallon (mpg) by 1965. The law set standards of 18 mpg for 1978 cars, 19 mpg for 1979 and 20 mpg for 1980. Under the law, mileage standards for 1981 through 1984 are to be set by the Secretary of Transportation.

Administration planners are considering using the 1975 mileage standards as a basis for the mileage efficiency tax.

Such a tax would boost efforts to force consumers to shift to better mileage cars, administration sources say. The existing law, which sets civil penalties for auto manufacturers who do not meet efficiency standards is not enough, they say.

The efficiency tax which would be an excise tax, would appear on sticker prices.

If Carter proposes an efficiency tax, he could face congressional opposition. Rep. John D. Dingell (D-Mich.), who chairs the House Commerce Subcommittee on Energy and Power, had a major hand in drafting the 1975 legislation that set existing standards. Dingell would be concerned about the impact an efficiency tax would have on the auto industry and autoworkers, a staff aide said.

The specter of tough mileage standards could raise even stiffer problems for Carter on the trade front. Foreign auto manufactures - especially the Germans and the Japanese - are already marketing cars that can meet stricter mileage standards than their American counterparts and are auxiously eyeing the lucrative U.S. auto market.

U.S. industry executives say the outlook for tough enforcement of mileage efficiency standards also could spur the development of American-manufactured diesel autos. Diesel fuels contain about 10 per cent more British Thermal Units (BTUs) than gasoline on a gallon-for-gallon basis. A BTU is the amount of heat necessary to increase the temperature of one pound of water one degree Fahrenheit.

General Motors Corp. is planning to introduce a diesel-powered car in its 1978 Oldsmobile line, and Ford Motor Co. plant to build diesel engines.

Administration sources say that they are considering a proposal that, through taxation, would effectively raise the price of natural gas used as boiler fuel to the equivalent price of imported oil. The tax would be based on the amount of BTUs in natural gas compared with the amount in oil. Last year almost 4 trillion of the 21 trillion cubic feet of natural gas consumed in the United States was used as boiler fuel.

A natural gas BTU tax, would have a heavy impact on the Southwest and the South, where natural gas is widely used as boiler fuel.

Revenues from a BTU tax could range from $3 billion to $5 billion.