Yes, vehicles have become more fuel efficient, but a just-released report by the Institute on Taxation and Economic Policy shows that inflation is 3.5 times more responsible for the decline in the purchasing power of the gas tax.
5 days ago Institute on Taxation and Economic Policy
Vermont's Transportation Secretary points to increased fuel efficiency as reason to look for an alternative revenue option, favoring Oregon's Road Usage Charge. Meanwhile, U.S. DOT reactivated its "ticker" to warn of funding cutoff after July 31.
Jun 23, 2015 VPR
Transportation for America recaps the first meeting in three years by the House Ways and Means Committee to address transportation funding. Chairman Paul Ryan decried the cumulative $63 bailout of the Highway Trust Fund but ruled out a gas tax hike.
Jun 21, 2015 T4America Blog
In his first substantive statements regarding his ticket's failings during the recent Presidential election, GOP leader Paul Ryan "is looking to the nation’s big cities" as the cause.
Nov 14, 2012 The New York Times