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A guide on how to operate vehicles is here.
The easiest way to stay alive is to prevent your enemies from getting the weapons that will severely hurt you. Rocket Launchers and the Spartan Laser are major items that you must grab, or at least ensure are controlled by your team. Also the Halo: Reach grenade launcher, and Halo 4's railgun and sticky detonator can pose serious threats as well.
Grenades are the major problem for a standard driver. Stay clear from any grenades to avoid damage to your vehicle. A flipped vehicle is a problem, but a practiced driver can press the left analog stick forward, turning the front wheels (or other steering mechanisms) to successfully roll out of the explosion. If you keep your cool, maintain your momentum, and steer out of the blast, your vehicle should land safely, and your joyride will continue. If forced out of your vehicle and surrounded by opponents, finishing off your vehicle with a grenade or two has a chance of taking some of them out with the resulting explosion (or at least weakening their shields) - a much better option than dying without causing some damage. Also, human ground vehicles feature a power brake, (LT), which can be used to prevent flipping.
The human vehicles are primarily land-based, which means that they struggle with rough ground less than their Covenant equivalent. Armed almost exclusively with ballistic weapons, they include the two fastest vehicles in the game capable of taking passengers.
The Warthog has room for the driver, gunner, and a passenger. It is ideal for getting into or out of an area quickly, be it with flag, bomb, ball or just getting to the hill. It is the driver's job to keep himself and the passengers alive, which is easier to do if you just keep moving - your enemy may have a good weapon but it will be much harder for him to hit you. The Spartan Laser charges for three seconds, which is plenty of time to turn behind a barricade or wall. Generally the best advice when using the Warthog for raiding or escaping with the flag is to take the lowest terrain available as it will prevent long distance shots against you, and to know which route you plan to take. Due to its tendency to fishtail it is not the easiest vehicle to drive but with practice this weakness can become another tool at your disposal - either for crushing opponents or for turning into a corner at high speed.
The gunner should ideally keep the gun firing at all times (due to the delay speeding up the barrel)(Note:Only applies in Halo 2 or Halo 3, as in Halo:Reach the chaingun will overheat if fired continuously) and concentrate their fire on individuals or light vehicles, unless using the Gauss Warthog, which is fairly effective at getting through armor on the heavier threats. Passengers will find themselves at a disadvantage, as the decreased field of fire makes it difficult to aim and hit accurately. Unless playing an objective game, the passenger seat should remain free as a single mistake by the driver can result in an easy triple-kill for a skilled opponent with grenades, Spartan Laser, or Rocket Launcher.
Rocket Hogs are a new variant of the Warthog introduced in Halo: Reach. The only difference is the weapon, a 6 round rocket fire. Effective against infantry and aircraft but is weak at close ranges and vehicles.
The Mongoose is a two-man ATV with a driver and a passenger, who has full 360-degree field of fire. As this vehicle mounts no weapons of its own, its only defense is speed and the passenger, who can fire a weapon from the rear. However, due to its small size it is much more maneuverable than the Warthog, capable of getting inside most buildings or caves. Tactics remain much the same. The Mongoose is effective in Objective games.
Scorpion tanks are by far the most coveted vehicles on Multiplayer maps and in Campaign. The Scorpion is a proverbial "One Man Army", more so than a Spartan. They house a chaingun attachment, as well as the typical HE shell. The shell round is most effective against vehicular opposition, but due to splash damage is equally effective against ground resistance. The only set back is a 2-3 second delay for "reloading", and the fact you have a minimum range. The chaingun works best in repelling potential boarders (something every good driver should be aware of) and taking down targets that are left after a vehicle's destruction. In Halo: Combat Evolved and Halo 2, the driver has access to both weapons. However, in Halo 3, the chaingun is now a separate weapon, operated by a separate gunner, similar to the Covenant Type-26 Wraith. This is both a blessing and a curse, as although it is useless without an ally, it can be aimed independently of the main gun.
The only real weakness of this vehicle is the incredibly slow speed, and the slow reload time of the main weapon. However these are easily offset by the power of the cannon, especially if you have an escort of Ghosts or Warthogs for protection. It should be noted, that the tank is faster than traveling on foot. In Halo: Combat Evolved, the driver of the tank is exposed in multiplayer, easily taken out by a sniper.
The Hornet consists of a one-man cockpit which makes up most of its mass, and a rear section extending a meter or so backward. From the top of this rear section are a set of wings with what appears to be a jet engine/turbo fan at the ends, providing the Hornet's main source of propulsion. It also seems to possess surveillance equipment on the nose, similar to that of the AH-64 Apache Helicopter. On each side of the cockpit is a jump seat extending backward (doubling as a landing skid), which allows up to 2 passengers to fire from the sides. It is slightly longer in length than the Covenant Banshee, and its wingspan is larger as well. Unlike the Banshee, the Hornet can remain stationary in the air. It also lacks the evasive maneuvers of the Banshee. The Hornet is used in the Halo 3 level The Covenant for aerial combat. Later in the mission they prove to be particularly useful in regards to taking down Scarabs as it can hover in position to destroy the power core, or remove any Brutes from the Scarab. The Hornet can also be used in custom games in Halo 3 multiplayer, and is available both as the Campaign-style "Attack Hornet" or "Transport Hornet," which possesses no rockets and weaker,alternating chainguns.
The Falcon is the Halo: Reach replacemet for the Hornet. It usually requires more than one person to be in an assault state. With 2 passengers the Falcon is a deadly machine that can present a fair fight even to a Banshee. Just make sure that once you find a good target you put it in one of you're gunners' line of site.
The Mantis is a powerful bipedal mech used in Halo 4. If a player encounters this vehicle it would be wise to immediately use it, or point it out to a more qualified player. The Mantis is equipped with rockets, a machine gun, and a melee-like stomp capability. the rockets are ideal for vehicle destruction, but are also extremely helpful for knocking of approaching infantry. If an opponent is approaching with a charged plasma pistol, or is about to through a grenade, the rockets should be your go to weapon, providing an almost immediate kill with just one accurately placed rocket. The machine gun is not terribly powerful, but nevertheless, surprisingly effective at eliminating enemies. The stomp attack is a very risky move, as it leaves the player immobile and vulnerable for a short period of time. This attack can often pay off, however, as it can quickly kill any opponent in just one stomp. This makes it ideal for those times when an opponent may step out in front of you with a charged/charging plasma pistol, rail-gun, or Spartan laser.
The Pelican, specifically the gunship variant, is only playable without a glitch or easter egg in one level: Halo 4's Shutdown. It has two weapon systems available to the pilot, the Spartan Laser and a heavy machine gun. The Spartan Laser is ideal for taking down any Phantoms that might get in the player's way. It is advisable to keep a long distance between the player's Pelican and enemy Phantoms, as the Spartan Laser's infinite range leaves the player's accuracy as the only limiting factor.
In this vehicle, you have the ability to shoot and control your vehicle, and so is the transport of choice for the solo player, or for escorting a fully loaded warthog in objective games. A good pilot will use a combination of both the plasma cannons and the Ghost's boost to get Splatters. Keep in mind that the Ghost has an excellent climbing ability and can climb walls easily to a certain extent.
The Covenant version of the Warthog, the Spectre has an enclosed cockpit, while the side seats are completely open, the passengers held in by a similar technology to the Covenant artificial gravity machines. In the rear is a turret, similar to the chaingun mounted on the back of the M12 LRV Warthog (as opposed to the Gauss Warthog). Unlike the Ghost, which has a smaller power source, the Spectre can boost while the gunner is firing. Compared to the Warthog, it is weaker, but trades armor for speed, like most of the Halo 2 Covenant vehicles compared to their human counterparts. The Spectre also has 1 more passenger seat than the 'Hog.
The Spectre was introduced in Halo 2, and does not appear in Halo 3. It is replaced by the Brute Prowler, which has a similar in-game function.
The Wraith is generally used as artillery, and for offensive attacks on fixed positions, such as in a CTF/Assault match. The plasma mortars have a noticeable arc to their trajectory and are also quite slow. The best tactic in a Wraith is to fire into heavy enemy concentrations, and use the boost feature to elude (or eliminate) potential boarders.
The Banshee is one of the most favorable vehicles on the map. A good Banshee pilot will be able to discern when it is a good time to retreat, although generally the 'weapon lock' beeps are a good indicator. The Banshee has the ability to dodge fire, though turrets and cannon fire are more of a challenge than Surface to Air Missiles. Take note of who the player with the rocket launcher is, and keep him in your sights. If the Rocket wielding player fires at you, you will be able to dodge the shot (A + Left Thumbstick) with the proper timing. As stated before, it is harder to dodge instantaneous fire from the Scorpion's main weapon and stationary turrets. This is curable by attempting to stay out of the opponent's reticule, constant change of direction and off-and-on boosting will help with this strategy. Using the dodge ability is not advisable when attempting to avoid instantaneous fire, as the dodge is predictable.
The Chopper is a Brute vehicle whose bulky and armored frontal section makes it useful for splattering and running over victims caught unawares. The Chopper has a limited boost capability in short spurts, but while at full boost speed, the Chopper can destroy all but the largest vehicles. Along with its devastating splattering powers, the Chopper boasts two autocannons. One drawback of this vehicle is its inability to strafe, as well as not being able to turn while stationary. It serves as the Brute counterpoint to the Ghost.
The Brute Prowler is a vehicle used by the Brutes as a troop transport in the Halo 3 Campaign. It is not found on any of the default variants of maps in Halo 3, but newer matchmaking variants have featured the vehicle. It is useful when you have many available allies to fill it with, as it has a very powerful turret and two spots for allies to ride on. Always have someone on the turret if you are using this vehicle, and if possible, arm your allies riding on the sides with powerful, anti vehicle weapons like the fuel rod gun, rocket launcher, and Spartan laser. Due to its unique shape and wings on the side, it is very difficult to flip.
The Revenant serves as a smaller, much more agile version of the Wraith. They have a similar plasma mortar, though far less effective. This vehicle serves as a better counterpart to the Warthog than the Ghost, but its maneuverability is almost as good. In general one must be careful not to stay still with this (and all) vehicle, as the open cockpit leaves one extremely vulnerable to snipers, and the weak design of the vehicle can be fairly easily destroyed by a few well placed rockets or grenades. Take caution also when attempting to splatter other players. The area affect of the mortar may be frustratingly small, but if the player has the armor lock ability he could severely damage, destroy, or even hijack your vehicle.