Newsflash, most taxes have loopholes in them to allow the money to be used on things other than what they were intended for [1]. Vehicle taxes are used for multiple purposes, not just roads. Currently, 7.2% of GDP goes to pensions [2], and that is just the federal government's part. States' contributions would easily double that figure.
[1] https://reason.org/policy-brief/how-much-gas-tax-money-state...
[2] https://www.usgovernmentspending.com/pensions_spending