ok here's your solution. states tax cars based as "property" with registration fees. if you look at the percentage of hybrids on the road a very slight bump on all registrations would cover the difference in what's lost in consumption or "at the pump" taxes, now the roads do have to be paid for and eventually when gas is the lesser used mode of fueling transportation a different solution would be necessary. by then the solution would be to shift the "gas tax" to the electric bill, or a tax on the power drawn at charging stations and such, still the inherent problem with taxes for road upkeep based on a tied to transportation its going to be hard to find an equitable solution with electric as the mounted battery seems to be the choice of the manufacturer (a mistake I think, here's why and what I think would be a workable model)
imagine all electric cars having a standard battery, same hookup same size etc. like a "d cell" in flashlights. cars would be built with a "cassette" battery access, or more like an 8 track where a car running low on power could pull into a service station, and the battery would be swapped out with a charged one for a fee plus the differential cost between the power in the old battery and the new just like topping off a tank of gas.
for this to work the batteries would have to be standardized and would also have to be mounted in such w way that they could be swapped relatively easily with say a lift of sorts that an attendant can operate to facilitate a quick change. THEN the power could be taxed and still allow the "station" to be competitive depending on how they received or generated the power they sold , be it solar or on the grid. transactions can be tracked. now, many folks would "plug in " at home to avoid the higher cost of a charge , but it would still catch a larger percentage than it will with the hard mounted battery setups of most cars today. AND it would eliminate one of the big issues with electrics, the worry about a battery going bad and not holding a charge as long with age. the stores would always have as many batteries to "sell" as they start with as they would get one when the sold one so no need for a user level core charge , the stations could pay a core to acquire the "starter" batteries and be able to return bad ones for replacement if they receive a bad one which could then be recycled