The average fuel efficiency of the fleet of new cars and trucks rose only slightly in 2008, but the government said Friday an increase in the sales of smaller vehicles due to high gas prices could push the numbers higher.

The Environmental Protection Agency reported that the average performance of new, 2008 model cars and trucks was 20.8 miles per gallon in 2008, up 0.2 mpg compared to 2007 model year vehicles a year ago, and a 1.5 mpg increase since 2004.

The estimates, which the agency uses on vehicle window stickers on dealer lots, are based on a combination of pre-sale road tests and projections of likely sales of the new model vehicles. But with people buying smaller, more fuel efficient vehicles — and fewer SUVs and pickups — those fleet-wide projections were likely to be off the mark, the EPA acknowledged.

The EPA said it was “extremely likely” the 20.8 mpg estimate for 2008 model vehicles is too low.

Honda Motor Co. led the way with a projected 23.6 mpg, followed by Toyota Motor Corp. with 23.4 mpg. Hyundai Motor Co. was third with 22.6 mpg.

Nissan Motor Co. and BMW AG had estimated fuel economies of 21.2 mpg and Volkswagen AG stood at 21 mpg.

Domestic automakers, which have sold a higher mix of trucks and SUVs, were led by General Motors Corp. with an estimated 19.6 mpg. Ford Motor Co. had an estimated 19 mpg and Chrysler LLC came in at 18.9 mpg.

Among individual vehicle segments, hybrids such as the Toyota Prius (46.2 mpg combined), Honda Civic Hybrid (42.9 mpg combined), and Ford Escape Hybrid front-wheel drive SUV (31.5 mpg combined) led their peers.

The EPA noted that the estimates were based on 2008 model year sales projections that were made when gas was selling at $2.50 to $3 a gallon, far below the $3.50 a gallon average during the sales period.

During the year, sales of midsize sport utility vehicles, large SUVs and large pickup trucks have fallen about 15 to 25 percent below the auto industry’s projections, EPA said.

Fuel-sipping subcompact, compact and midsize cars, meanwhile, have become more popular and vehicles with 4-cylinder engines have gained market share during the span. EPA said the combined factors could lead to higher fuel economy estimates when more sales data is available.

President Bush signed an energy bill last year that requires car makers to meet a fleetwide average of at least 35 miles per gallon by 2020, a 40 percent increase over current standards. Those figures are derived from laboratory testing and the EPA said that average increased to 26 mpg in 2008 compared with 25.7 mpg in 2007.

Environmental groups said the report reflected a lack of good options among many vehicle categories. In the larger truck segments, for example, there was little difference in fuel efficiency among the best performers and the mediocre ones.

“The American people have not had the choice to buy a clean vehicle except for certain sized sedans and the smallest SUVs,” said Dan Becker, director of the Safe Climate Campaign for the Center for Auto Safety.

But Charles Territo, a spokesman for the Alliance of Automobile Manufacturers, which represents GM, Toyota, Ford, Chrysler and others, said the report showed the lengthy list of fuel-efficient vehicles on the market.

More than 100 vehicle models offer 30 mpg or better on the highway and they “have become extremely popular recently with the rise in higher gas prices,” he said.

Fuel efficiency has become a top selling point at dealerships with the rise in gas prices.

Projected U.S. auto fuel efficiency for 2008 model vehicles nudged higher, the Environmental Protection Agency said on Friday, but cautioned its estimate was likely low due to sharply higher fuel prices and volatile vehicle sales.

The EPA, which annually calculates expected fuel performance for the entire fleet, said the current projection for 2008 models is 20.8 miles per gallon compared to 20.6 for the year ago period.

“It is extremely likely,” the agency said, that the estimated average is “too low” and would be revised next year once the degree of industry volatility is known.

Nevertheless, EPA officials pointed out the 20.8 mpg figure is the highest since 1993 and stressed that its partial analysis did not contradict incremental gains experienced since 2004.

“This report shows we’re driving in the right direction,” EPA Administrator Stephen Johnson said.

But the Union of Concerned Scientists, called the EPA assumption outdated and said government and industry could be more aggressive on efficiency in the near term.

“Consumers are doing the best they can to buy more fuel efficient cars and trucks, but automakers are not providing enough options in showroom floors,” said Jim Kliesch, senior engineer for the environmental group.

The EPA said the 2008 efficiency calculations do not fully reflect the dramatic jump in gasoline prices this summer or the corresponding slump in sales, especially for Detroit-based manufacturers whose product lines are heavy on less efficient sport utilities and pickup trucks.

U.S. auto sales were down more than 11 percent overall through the first eight months of 2008 compared with a year earlier as tight credit markets, a weak economy and rising gasoline prices pressured demand for trucks and SUVs.

Sales data so far suggest that subcompact, compact and midsize cars are the only classes to have met or exceeded sales projections with gas prices averaging $3.50 per gallon this year, EPA said.

Vehicle product plans were submitted to the government with gasoline price ranges of between $2.50 and $3 per gallon, the EPA said.

An EPA spokeswoman said a final figure for the 2008 model year may not be published until mid- to late-2009.

The inconclusive EPA report is apt to further muddle the picture for regulators working to set longer-range fuel efficiency targets for industry.

The National Highway Traffic Safety Administration (NHTSA) has been criticized by environmental groups and some members of Congress for drafting a plan for 2011-2015 that relies on outdated gasoline price assumptions. NHTSA uses EPA data in its calculations.

Struggling Detroit automakers are lobbying Congress to extend $25 billion in low interest loans to the industry to help it make more fuel efficient vehicles and meet a federal mandate for 40 percent better fuel efficiency by 2020.

Hybrids led by Japanese manufacturers are the most efficient in the mass market but U.S. manufacturers have made progress, according to product plans submitted to environmental regulators.