Every vehicle has a baseline health value as well as baseline damage values for its various abilities. When a player mounts the vehicle, these values are modified according to the gear the player has equipped.
The game calculates a modifier based on the unweighted
Item level average of currently equipped gear. A driver's modifier affects vehicle health and damage of driver abilities. A gunner's modifier affects damage of gunner abilities but has no effect on vehicle health. The modifier is calculated at the time you mount the vehicle. If a player decides to do gear swaps (for the purpose of increasing your modifier), then the player should eject and remount the vehicle in order for the new modifier to take effect.