Roughly half, though this varies widely and is prone to differences of opinion on what counts as paying for roads.

https://en.wikipedia.org/wiki/Fuel_taxes_in_the_United_State...

https://taxfoundation.org/gasoline-taxes-and-user-fees-pay-o...

Aside from the fact that this isn't really true, you have to also account for many externalities that car dominance creates:

- Land use. While every transportation mode uses land, cars use way more, and in dense urban areas that land is very valuable; users of the land who are driving are getting it 'for free', when it could go to a more productive purpose.

- Pollution

- Noise

- Danger

All of these are 'costs' that just looking at how much money you spend on roads won't account for.