The fuel economy and carbon-dioxide emissions of vehicles sold in the U.S. showed improvement in the ’09 model year and regulators expect efficiency to climb again in ’10.

Ironically, record-low sales have contributed to the gains.

The Environmental Protection Agency also says in a new report it expects Hyundai Motor America to leapfrog Toyota Motor Corp. as the nation’s cleanest, most fuel-efficient seller of ’10 vehicles.

EPA data projects Hyundai’s fuel economy and emissions at an industry-best 25.9 mpg (9.1 L/100 km) and 343 grams per mile, up from a second-best 25.1 mpg (9.4 L/100 km) and 355 g/pm in ’09.

Toyota likely will see its fuel economy and emissions worsen to 24.5 mpg (9.6 L/100 km) and 363 g/pm in ’10, down from 25.4 mpg (9.3 L/100 km) and 349 g/pm in ’09.

The ratings would sink the auto maker, which unlike Hyundai sells trucks, from No.1 on the EPA’s efficiency chart to No.5.

The EPA numbers for ’10 are preliminary and based on confidential pre-model-year production volume projections from auto makers, so changes in the vehicle mix driven by fuel prices and the economy could affect the final data due out next year.

In addition, fuel economy figures from the EPA are adjusted to reflect its “real-world” testing cycle and are the numbers appearing on new-vehicle labels.

Laboratory figures – or the numbers the National Highway Traffic Safety Admin. uses to calculate an auto maker’s compliance with corporate average fuel economy rules – generally are about 25% higher because they do not include EPA credits for items such as flexible-fuel vehicles.

Auto makers selling in the U.S. are marching toward a fleet-wide average fuel economy of 34.1 mpg (6.9 L/100 km) and emissions of 250 g/pm for ’16. The goal would equal a CAFE target of 35.5 mpg (6.6 L/100 km).

Toyota blames its efficiency declines on a shift in its’10 sales mix to less-efficient trucks. On the car side, it continues to book gains among the industry’s leaders.

“Toyota remains consistently above the industry average, and the trend is to improving fuel economy,” Toyota spokeswoman Cindy Knight tells Ward’s. “The (’10) numbers are preliminary. We’ll see where sales finish out.”

The Toyota Prius also retains the crown as the most fuel-efficient vehicle on the road, averaging 49.4 mpg (4.8 L/100 km), according to the EPA.

Hyundai aims to become the most-efficient U.S. auto maker. Earlier this year, HMA CEO John Krafcik made the bold prediction of 50 mpg (4.7 L/100-km) for its fleet by 2025, which accounting for the difference in the EPA test cycle equals roughly 37 mpg (6.4 L/100 km).

The five most-efficient auto makers in the U.S. for ’10 also include Honda Motor Co. Ltd., projected to finish second; followed by Kia Motors Corp. and Volkswagen AG. General Motors Co., Ford Motor Co. and Chrysler Group LLC rank among the bottom four car companies.

Fleet-wide, fuel economy and emissions are expected to improve in ’10 to 22.5 mpg (10.4 L/100 km) and 395 g/pm, up from 22.4 mpg (10.5 L/100 km) and 397 g/pm in ’09.

The ’09 values represent the best performance by auto makers since ’87, with both fuel economy and emissions improving 2% over their previous high. After ’87, fuel economy and emissions worsened until ’04. Between then and ’09 the values have improved 16% and 14%, respectively.

The EPA also says the ’09 record owes partly to poor economic conditions that led to the lowest vehicle production since the database began in 1975 with the enactment of CAFE. Truck production in ’09 fell 8% to 40% of the mix, down 12% from its peak in ’04 and at its lowest level since ’95.

While the EPA expects Toyota’s efficiency to decline in ’10, the auto maker booked the greatest improvement in fuel economy and emissions between ’08 and ’09. Nissan Motor Co. Ltd. and Ford posted the next greatest improvement between the model years.

Chrysler was the only auto maker that failed to improve fuel economy and emissions between ’08 and ’09, and ranks as the worst performer of 14 of the biggest vehicle manufacturers selling in the U.S. It will take the title again in ’10, the EPA predicts.

Further reflecting fewer trucks in the production mix, the average weight of an ’09 vehicle fell to 3,917 lbs. (1,706 kg), the lowest level since ’01 and a reduction of 14%, or 168 lbs. (73 kg), from ’08. The EPA expects ’10 cars and trucks to gain weight, adding some 92 lbs. (40 kg).

Horsepower between ’08 and ’09 declined 5% to 208 hp from 218 hp, marking the largest annual decrease since ’08. But that figure will tick back up to an average of 220 hp in ’10.

The EPA also expects trucks to account for 41% of the mix in ’10, contributing to an increase in sales of vehicles with V-8 engines to 16% from 12% in ’09. Penetration of 4-cyl. motors in ’10 will shrink to 48% from 51% in ’09, after rocketing up from 38% of the mix in ’08.

Market share of 6-cyl. engines in ’10 will continue to remain flat at about 35% of the mix after commanding a majority share in ’08.

Advanced technologies will continue to rise in ’10, with variable-valve timing appearing on 86% of vehicles, up from 72% in ’09 and 58% in ’08. Gasoline direct injection will appear on 8.5% of ’10 models, the EPA projects, after comprising 4.2% of the fleet in ’09 and 2.3% in ’08.

Hybrids will increase their share of the U.S. market to 4.3% in ’10, up from 2.3% in ’09 and 2.5% in ’08. Diesel penetration will remain slim, with 0.4% of the market in ’10, off slightly from 0.5% in ’09 but up from 0.1% in ’08.