CHICAGO - U.S. environmental regulators are expected within days to propose rules to make trucks more fuel efficient, and trucking industry executives and lobbyists familiar with the process said the rules will probably call for boosting fuel efficiency by 2027 nearly 40 percent from 2010 levels. Truckers say the industry is willing to accept tighter federal standards, since motor fuel accounts for about a third of its costs. Truckers also want consistent standards throughout the country instead of a separate state rule in California. But various segments of the trucking industry disagree about how federal rules should be structured and implemented. So the Environmental Protection Agency proposal for heavy trucks could prompt an intramural struggle to influence the final regulations. The trucking rules are part of a broader effort by the Obama administration to curb greenhouse gas emissions. The EPA has said it intends to propose “performance-based” standards, allowing truck makers and operators to hit the target either through the engine, aerodynamic add-ons to trucks and trailers or software to make driving more fuel efficient. But manufacturers disagree about whether the overall standard should encompass the engine, the truck itself and the trailer, or whether it should set separate standards for each of the three main components of an 18-wheeler freight hauler. Companies like Cummins Inc and Eaton Corp want separate standards, especially for engines as they produce the emissions and have been tested for decades. But the U.S. trucking units of Daimler AG and Volvo AB want emissions measured for the truck as a whole. “We oppose a separate engine standard, which we believe is redundant because improvements in engine efficiency would be reflected in a complete vehicle assessment,” said Steve Berry, director of regulatory affairs for Volvo Group North America. “An overly stringent engine standard also could force manufacturers to introduce technologies and design changes before they are fully ready to do so.” An EPA spokeswoman said the agency could not discuss details of the pending proposal, since it was still under review. The industry expects targets close to the 40 percent fleetwide reductions sought by environmental groups and 46 percent for long-haul trucks by 2025. “The industry is expecting relatively stringent standards roughly in line with the targets of the environmental groups,” said Mihai Doborantu, director of technology planning and government affairs at Eaton. Industry executives say California regulators have pushed for strong federal truck emissions rules, so trucking companies are prepared to accept demanding federal standards to avoid California pushing its own separate standards. David Clegern, a spokesman for the California Air Resources Board, said the state “would be very supportive of a national standard” but the agency could not discuss the matter until the EPA proposal is issued. “Everyone from the federal government to the (truck) manufacturers to fleet owners and the state of California wants to see a national standard,” said a manufacturing executive who spoke on condition of anonymity. “For that to happen, California has to sign on and the EPA has to find that sweet spot.” The proposed rules may also spell out fuel options for vehicles such as garbage trucks. These usually run at low speeds and stop often, so they gain little from aerodynamic improvements but are candidates for alternatives to conventional diesel engines. Kerry Kelly, a senior director for federal affairs at Waste Management Inc said the company expects new rules will focus short-term on natural gas vehicles and longer-term on hybrids. Around 90 percent of Waste Management’s new vehicles run on natural gas, which produce 20 percent fewer emissions than conventional engines. So far, testing on hybrids has “brought mixed results” and the new rules are expected to allow time for more testing, Kelly said.