According to a Reuters report, The U.S. government plans to complete rules "soon" that significantly boost automobile efficiency, despite calls from some Republicans for further evaluation of the regulations. The Obama administration had planned to finalize standards last week that would require companies to reach an average fuel efficiency across their U.S. fleets of 54.5 miles per gallon by 2025, but the release of the regulations was delayed.

While declining to provide a specific timeline, the Transportation Department stressed the rules are moving forward. "The rule is still undergoing interagency review and we expect that process to be completed soon," department spokeswoman Lynda Tran said in a statement.

Some Republican lawmakers, led by House Oversight Committee Chairman Darrell Issa, have raised concerns about the fuel economy proposal that was crafted after months of negotiations between the Obama administration and auto makers. The process used to arrive at these new efficiency targets was flawed, lacked transparency and failed to adequately assess how the new rules would affect safety and vehicles costs, Issa and committee colleagues Jim Jordan and Mike Kelly said on Tuesday.

"We strongly urge you to utilize your authority to return the proposed rule to the Environmental Protection Agency and the (Transportation Department) for further consideration," the lawmakers said in a letter to Boris Bershteyn, who heads the White House's Office of Information and Regulatory Affairs.

The administration has disputed these claims, saying its policies will nearly double fuel efficiency for cars and save families thousands of dollars at the pump. If implemented as proposed, the rules would be a big jump from current standards that require auto makers to achieve 35.5 mpg by 2016.