The Obama administration is expected to issue the last rule in the president's far-reaching climate change agenda as soon as Tuesday, setting new greenhouse gas and fuel efficiency standards for big-rig trucks.

That may be the reason the president used his Saturday radio address to underscore the new rules in discussing the steps he will be taking to combat global warming, which he called "one of the most urgent challenges of our time."

"But we're not done yet," when it comes to taking action to reduce greenhouse gas emissions, he said. "In the weeks and months ahead, we'll release a second round of fuel efficiency standards for heavy-duty vehicles," as well as taking steps with Canada and Mexico to achieve a 50-percent clean energy goal for the continent by 2025.

Mexico is planning to issue its own rules for big trucks, and Obama has agreed to harmonize the forthcoming U.S. truck standards with Canada.

The final truck rule is phase two of the Environmental Protection Agency's joint program with the Department of Transportation to cut greenhouse gas emissions from heavy-duty trucks beginning in 2019. Phase one of the joint climate and efficiency rules end for model-year 2018 vehicles.

The new regulations were proposed last year and after next week will become law to be implemented under the next president's watch.

The truck rules establish standards to improve fuel efficiency and to reduce greenhouse gas emissions. Many scientists blame the emissions from burning fossil fuels for warming the Earth's climate, resulting in more droughts and flooding.

Big-rig trucks account for almost 20 percent of the nation's greenhouse gas emissions and oil consumption while only making up about 5 percent of all vehicles on the road.

The rules are expected to be issued next week with most of the large trucking fleets' blessing. Yet, some in the industry are worried about added measures that will increase the stringency of the rules and make them more expensive to meet.

Cummins, the largest engine maker in the world for big trucks, is supporting the rules, despite reservations it is hearing from some, a senior official with the company told the Washington Examiner.

"We have worked very closely with the regulators and other manufacturers," said Brian Mormino, director of energy policy and environmental compliance with Cummins. "All of that has gone in the right direction."

"The biggest change that I think is gaining attention out there is an overarching" change in the engine efficiency standard, he said. The proposed rule would improve efficiency by 4.2 percent, but there is "discussion it will move up to 6 percent."

Some say "this is awful," but Cummins is not worried, he said. Mormino said his company has the technology "to meet or exceed" the standards, and "that viewpoint does not decrease with this change." He said the rules are expected to be issued Tuesday.

Major truck fleet operators are preparing to support the regulation, say officials close to the companies.

A trade association representing major technology developers for heavy-duty trucks, Calstart, issued a report Thursday saying the phase-one standards have made the U.S. vehicle tech sector a global leader. The sector is set up to meet the requirements of the new rules, while boosting jobs in parts of the country that have been struggling, Calstart said.

The paper is the first-ever attempt to gauge the scope of the U.S. high-efficiency technology sector for heavy-duty trucks and buses, the group said. "This report showcases what we know from experience: the U.S. is a leader in transportation tech, including next-generation advanced engines," said David Johnson, president and CEO of Achates Power and a member of Calstart. "Our industry can meet aggressive efficiency targets at affordable prices. Strong standards will encourage continued investment and drive advanced tech innovation and jobs in this sector."

The heavy-duty rules are expected to contain a regulatory fix unrelated to big rigs, which the motorsport industry is desperate to see made.

The EPA had slipped a change into the truck proposal that would essentially ban vehicles modified for racing under the Clean Air Act. The EPA says it plans to remove the measure, but the industry isn't completely at ease.

The racing parts industry's lead trade group, the Specialty Equipment Market Association, is pressing Congress to support legislation to permanently stop EPA from ever imposing regulations on motorsports in the future.