Does stopping the car at the graves without dismounting and entering them count as a visit to them?

If one stands at the graveyard such that the graves are visible and their occupants can hear your salutations to them, then this counts as a visit for which you will be rewarded. If you offer salutations from a distance, there is no harm in it, provided you are close enough for them to hear. It is sunnah to offer salutations to the occupants of graves, whether you are passing by or standing. However, standing is preferable. If there is a wall and you cannot see the graves, it does not count as a visit.