Wheelman

Wheelman is a Halo 3 and Halo Reach Multiplayer Medal awarded and achieved when the player, as the driver of a vehicle, has one of their passengers kill an opponent. The design of the medal is very similar to the Vehicular Manslaughter Medal. The Wheelman medal actually takes on the design of the Halo 2 Roadkill Medal.

It is represented by a gold star with a tire on it. On the tire, it says 'ROAD KILL TIRE' twice.

Usually, it is accompanied by the message, "Your driving assisted ."

It can be achieved easily with any map that has a Warthog, especially during big team battle games as more maps in big team have Warthogs.

Strategies
Keep moving. If you stop a Warthog, Mongoose, Prowler, etc. it is a sitting duck, and can be easily stuck or hijacked.

When playing a slayer game only have a driver and a gunner. Having a passenger most likely will give the other team a Triple Kill. Only have a passenger if it is an assault or capture the flag game.

Trivia
It is in fact possible to get a Wheelman medal after someone sticks a player and then boards in a vehicle. Less common is filling someone up with needles or spikes and then getting in, as the time for the projectiles to explode is minimal.