Fallout Tactics features many vehicles that the player can operate. The cars in the game do not appear to actually use gas as there are no emissions of any kind. Fallout Tactics is set in what is the heart land of the automotive industry, giving some argument to the plausible presence of functioning vehicles. The vehicles are primarily used by the Brotherhood of Steel, who are among the few factions to have the technical know-how how to get vehicles functioning and maintain them (and they make sure it stays that way, by recruiting adept mechanics, eg. Casey Barrett).
When taking a vehicle to a mission location, it will not appear with you during the mission. Once you leave the mission, the next time you enter a location (either it be a base or random encounter) you will appear standing next to the vehicle with the engine still running, which is the only time a vehicle's ignition will ever be on without a driver inside.