Vehicles you control become artifact creatures until end of turn. Creatures you control get +2/+0 until end of turn.
Vehicles you control become artifact creatures and then also get +2/+0.
The set of Vehicles and creatures affected by Start Your Engines is determined as the spell resolves. Creatures you begin to control later in the turn wonâ€™t get +2/+0, and Vehicles you begin to control later in the turn wonâ€™t automatically become creatures and wonâ€™t get +2/+0.