(Updates with automaker group comment)

WASHINGTON, Aug 22 (Reuters) - U.S. regulators on Monday denied a request by major automakers to extend the comment period on environmental analysis to determine whether government fuel efficiency requirements are feasible through 2025.

The U.S. Environmental Protection Agency and the National Highway Traffic Safety Administration said in a July report that automakers have the technology to meet aggressive mandates to hike fuel efficiency, but improvements will not be as great as the Obama administration once forecast because buyers are switching to pickup trucks and SUVs.

Automakers had asked to extend the deadline for commenting on the 1200-page report until at least late November. Regulators said comments must be filed by late September.

Automakers have sounded alarms that low gas prices make the Obama administration's mandates to cut vehicle greenhouse gas emissions untenable in their current form.

The Alliance of Automobile Manufacturers, including General Motors Co, Ford Motor Co, Toyota Motor Corp , Volkswagen AG and Daimler AG, asked regulators on Aug. 1 to extend the comment period citing the sheer volume of information and other issues.

The U.S. Chamber of Commerce, National Automobile Dealers Association, Association of Global Automakers representing other major automakers, including Honda Motor Co and Nissan Motor Co, and other groups joined the Auto Alliance in filing a request for an extension.

Wade Newton, a spokesman for the Auto Alliance, said Monday the "comment period must be long enough to provide time for the public to fully understand the information that it has taken the agencies years to assimilate."

EPA spokesman Nick Conger said on Monday that despite denying the request, the agencies "will continue to consider relevant new data and information and welcome ongoing feedback as we continue to update our assessments."

The report will frame a debate with the auto industry that will be decided in 2018 by the next president.

Administration officials say an important finding of their analysis is that automakers can comply with the mandates using known technology, and deliver benefits in terms of fuel savings and greenhouse emissions cuts that outweigh the estimated $894 to $1,245 per vehicle in costs.

Trucks, which generate the bulk of the profits earned by Detroit's three, unionized automakers, are key.

When the administration first outlined its goal of boosting average fleet fuel economy to 54.5 miles per gallon, regulators forecast that 67 percent of vehicles sold in 2025 would be cars.