Plug-in hybrid vehicles are big fuel-savers. But there won’t be enough of them on the road to help automakers boost their fuel economy averages anytime soon, according to General Motors Corp., AutoTech Daily reports.

The Detroit Free Press says GM has made that point to federal regulators by way of cautioning them not to count on plug-ins when determining the feasibility of higher corporate average fuel economy standards.

The National Highway Traffic Safety Administration has proposed boosting fuel economy standards 25 percent by 2015. But it also asked OEMs to report their plans for plug-ins by the end of this year to help them estimate the viability of the new rule.

GM says plug-ins such as its upcoming Chevrolet Volt won’t be produced in significant volumes until sometime after 2015. GM has said it plans to launch the Volt in the U.S. in late 2010, make 10,000 vehicles in 2011 and 60,000 the following year. The latter volume is less than 2 percent of GM’s total U.S. sales last year.

GM’s stance contrasts with its vigorous efforts to generate positive publicity for the Volt as a breakthrough car to improve fuel efficiency and overtake Toyota Motor Corp. as the auto industry’s environmental leader.