Firing a vehicle’s weapon requires an attack action and uses the driver’s or gunner’s ranged attack modifier. A driver with 5 or more ranks in the Drive skill gains a +2 synergy bonus when firing vehicle-mounted weapons while driving.
Driving Defensively: Just as in melee combat, one can fight defensively while driving a vehicle, which grants a +2 dodge bonus to the vehicle’s Defense and applies a –4 penalty on attack rolls made by occupants of the vehicle. Total Defense: A driver can choose the total defense action which grants a +4 dodge bonus to Defense but does not allow the driver to attack (gunners or passengers take a –8 penalty on attack rolls). These modifiers last until the driver’s next round of actions.
Gunners or passengers, however, can take full attack actions, since they don’t have to use a move action (except, perhaps, to change positions in the vehicle). In general, taking a full attack action is useful only if a character has a base attack bonus high enough to get multiple attacks. A passenger can make multiple attacks with his or her own weapon. A gunner can make multiple attacks with one or more weapons controlled from his or her position.