Virginia's gasoline tax brought in 6 percent less for the number of miles that vehicles traveled in a year due to higher fuel efficiency, creating a growing gap in the commonwealth's transportation funding, according to Virginia's top transportation official.

Virginia Transportation Secretary Sean Connaughton said Wednesday at a policy luncheon hosted by Politico Pro that the gas tax revenues per vehicle miles traveled dropped statewide as more alternative vehicles such as hybrid and all-electric vehicles have taken to the roads.

The gap will continue to widen as federal regulations call for fuel economy to reach an average 54.5 miles per gallon by 2025 for cars and light-duty trucks.

"The foundation of our funding is very shaky," Connaughton said.

The number of vehicle miles traveled statewide in fiscal 2010 climbed 13 percent, according to state figures, but gas tax revenue grew only 5.8 percent over the previous year as vehicles were using less fuel to travel those miles. The revenue per vehicle miles traveled dropped 6.4 percent. The year before, data show, the decrease was 3.9 percent. State officials did not have updated data for more recent years but expect the trend to have continued in fiscal years 2011 and 2012.

That means the roads may be getting more crowded but motorists are contributing a smaller share of gas taxes that fund road and transit improvements.

Virginia officials are trying to find ways to fix the widening gap and boost the state's transportation funding. Gov. Bob McDonnell is working on a package that Connaughton said would bring in at least $500 million in additional revenue each year. The Republican governor has mentioned the possibility of indexing the gas tax to inflation, a proposal that raised the ire of anti-tax activist Grover Norquist this week.

The commonwealth currently charges 17.5 cents for every gallon of gasoline sold, a rate that hasn't changed since 1986.

Connaughton said the governor is meeting with people, including Norquist, for suggestions on how to raise the money. He said the governor expects to submit a final package in about three weeks.