All I had to do was google “Are roads fully funded by the gas tax” to find a contracticting source:

> Nearly as much of the cost of building and maintaining highways now comes from general taxes such as income and sales taxes (plus additional federal debt) as comes from gasoline taxes or other “user fees” on drivers. General taxes accounted for $69 billion of highway spending in 2012.

[pdf] https://uspirg.org/sites/pirg/files/reports/Who%20Pays%20for...