Consumer Price Index (CPI)

New Hampshire will consider a Republican bill in 2014 to tie the gas tax to inflation, increasing it by about 4 cents next year should it pass. The Senate killed a House bill in May to increase the gas tax by 12 cents over 3 years.
Dec 9, 2013   Concord Monitor
MAP-21 expires on September 30, 2014. The following year, the Highway Trust Fund will be insolvent according to the CBO. Alternatives involve user fees paid by motorists: the gas tax and VMT fee, and two others that bear no connection to driving.
Jun 11, 2013   Politico Pro
Now that Maryland has joined Wyoming in increasing its gas tax, who's next? Gas tax legislation in New Hampshire and Vermont have advanced to their Senates where going is rough, and a new, "two-cent a year for a decade" bill is proposed for Nevada.
Apr 10, 2013   The Washington Post
It's not too early to think about "fixing the Highway Trust Fund", a prerequisite to renewing MAP-21 transportation legislation that expires Sept. 30, 2014. Rep. Peter DeFazio may have the fix in the form of an indexed gas tax - but which index?
Apr 4, 2013   American Society Of Civil Engineers