The rapid growth in the number of cars and trucks worldwide threatens to wipe out any environmental gains that come from having those vehicles burn less fuel in a cleaner manner, according to an environmental report released yesterday.

The study by the World Resources Institute, a well-respected Washington-based research-oriented think tank concerned with environmental matters, adds yet another voice to an increasingly loud debate on the best ways to control the emissions from cars, trucks and buses. Together, these sources account for about 14 percent of the world's fuel-related carbon-dioxide emissions.

The outcome of that debate could determine the nature and cost of public and private transportation in the future and the success of attempts to slow the global warming that many scientists believe threatens profound changes in the world's environment.

According to the report, if current trends continue, the amount of carbon dioxide produced by motor vehicles will increase by 50 percent in the next 20 years. Carbon dioxide is one of the so-called "greenhouse gases" that have been identified as contributing to global warming.

At the heart of the institute's report is the finding that fuel consumption and the pollution it produces has continued to grow, despite improvements in fuel efficiency. In the United States, for example, the amount of fuel used per vehicle has declined 20 percent between 1970 and 1989; but overall fuel consumption increased 40 percent during that period.

The problem is a population boom in the number of cars, buses and trucks worldwide and the increase in the number of miles those vehicles are driven, the report said.

The number of cars in the world has grown from approximately 53 million in 1950 to more than 500 million today. While the growth is expected to level off in the United States and other industrial nations, that will be more than offset by rapid growth in other parts of the world, according to the study. By the year 2010, the number of vehicles could easily double, the report found.

Improvements in fuel efficiency "have been overwhelmed by growth," said James J. Mackenzie, who authored the report along with Michael P. Walsh. Although they said that there have been substantial conservation and environmental benefits from improved fuel efficiency, "the very real benefits from such improvements will eventually be canceled out by the impact of more vehicles being driven more miles," according to the report.

Automakers have made the same argument to attack the benefits of further improving fuel efficiency. Current law requires average fleet fuel economy of 27.5 miles per gallon, but Sen. Richard H. Bryan (D-Nev.), is pushing to boost that to 40 miles per gallon by the year 2001.

MacKenzie and Walsh noted, as the carmakers have, that increased fuel efficiency, combined with relatively low gasoline prices, has led to increased driving and higher gasoline consumption. However, the authors concluded that tougher fuel economy standards are needed along with higher fuel prices to help control consumption. Also needed are programs to improve mass transportation systems and to develop non-fossil fuels for private and public-use vehicles, the authors said.

Automakers had no immediate response to the report, saying that they would withhold comment until they have had a chance to review the study.