Earlier quoted context omitted.
That's misleading -- yes, in the aggregate, gas taxes cover only part of the expenses, but that's not because of personal cars . Damage on the roads -- and thus maintenance costs -- scales with the 4th power of weight per axle. So the damage is virtually all from large trucks, with sedans and SUVs as a rounding error. If you were to apportion out the maintenance cost due to personal cars, they are overpaying.
That's not really the view from a city budget. Cities don't have that many truck routes and their costs are dominated by maintaining streets where virtually all the traffic is light vehicles. For the state it is a bigger problem because almost all state route are truck routes.
[1] If your argument is that these are all 'light' vehicles, my city disagrees with you. They recently cited road damage from the variety of garbage hauling services as a reason to limit our options. Not that I entirely buy that as the only or even main reason they did so... local politics and all that.
[2] However they have been known to use secondary streets when they shouldn't.