Copypasted from wikipedia's Road Tax article:
United States
Each state requires an annual registration fee which varies from state to state.[5]
For example, in Massachusetts, the excise tax is billed separately from registration fees, by the town or city in which the vehicle is registered, and was set at a fixed rate of 2.5% statewide by a 1980 law called Proposition 2½. Within some states, the fees may vary from county to county, as some counties have surcharges per vehicle. An example of this is Virginia's personal property tax. The state of New York, on the other hand, charges a tax based on the vehicle's weight, rather than on its value, which is charged at the time of registration renewal.
In California, the registration tax is calculated by the current value of the vehicle. As a result, older and more inexpensive vehicles will have a low registration fee, whereas newer and more expensive vehicles will have fees in the hundreds of dollars.
So you do have some sort of road tax. You sure road tax isn't the fee you pay for the tag? Or am I off?