The staff on the Federal Trade Commission has proposed sweeping changes in the kind of automobile advertising that may appear by placing new restrictions on the type of fuel economy automakers may claim for their new vehicles.

The FTC staff proposal, which comes only a few weeks after the Environmental Protection Agency announced a revised method of evaluating "estimated miles per gallon," will go before the full commission this week.

Although the proposed new rules fall short of prohibiting automakers from using their own estimates of a car's fuel efficiency, they mandate that the new EPS "estimated mpg" - which has replaced the old highway-city mileage figures - must be displayed "with substantially more prominence" than any other estimates.

The EPA recently changed its system from a highway-city mileage breakdown to what it called a "more realistic" single rating that is close to the former city mileage figure. The EPA contended that the old highway figure were substantially higher than the actual mileage figures of most consumers' vehicles.

However, publication of the old figures in updated form was continued by EPA, which could not prevent automobile advertising from including the old figures.

The FTC, where jurisdiction covers advertising restrictions, had received in recent weeks several proposals ranging from banning all but the new EPA figure to allowing various estimates with certain disclosures.

The commission start proposal urged elimination of a previously required statement saying that the actual mileage achieved by a new vehicle may vary from EPA estimates.

It would be replaced by "disclosure of three of the variables in the testing process most likely to affect mileage performance: driving speed, trip length and weather conditions."

Disclosure of those variables "should help consumers adjust their expectations about the applicability of the EPA estimates," the staff said.

The FTC staff complained that most automobile ads since 1975 "have presented estimates for only the most economical vehicle configuration of the model advertised." The staff also noted that "the engine size disclosures have tended to be unreadable.

"Mileage estimates measured for vehicles with more efficient configurations should not be presented so as to give the impression that those numbers apply to less efficient versions of the same model," the staff said.

"For example," it said. "If an advertisement states that two engine sizes are available for a given car but provides only one "estimated mpg." the ad should make clear the engine size to which the EPA estimate applies."

Because it was assumed that the FTC staff would propose to ban any non-EPA mileage figures from advertising, the staff recommendation to allow other mileage estimates was surprising to industry observers.

"Considering the question raised about the reliability of EPA highway and combined estimates, and in light of several comments it has received, the (commission staff) has determined that non-EPA numbers should be permitted," the staff noted.

While it does not plan to allow "unreliable, improperly qualified figures," the staff proposal added that it "does not want to discourage the development of new more, reliable methods of measuring fuel economy."