The "fair share" most small cars - especially fuel efficiency focused designs that remove as much mass as possible - should be paying approximately $0. Road damage per-axel is proportional to the weight carried by the axel raised to the fourth power! A fully loaded 5 axel tractor trailer does thousands of times more damage than one of the small electric cars they are worried about taxing.
The "fair" solution where maintenance costs are paid in proportion to the actual sources of damage would be to ignore most carts entirely and raising the registration fees on the vehicle actually damaging the roads the heavy trucks. As vehicle weight grows, their responsibility for maintenance costs exponentially.