From Halopedia, the Halo wiki
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Wheelman is a Halo 3, Halo Reach, Halo 4 and Halo 5: Guardians multiplayer, Firefight and Spartan Ops medal awarded when the player, as the driver of a vehicle, has one of their passengers kill an opponent. In the first two games it is represented by a gold star with a tire on it. Writing on the tire reads 'ROAD KILL TIRES'. In Halo 4 it is represented by a white skull with a steering wheel on the forehead atop a blue circle and grey pentagon. In Halo 5: Guardians the skull and steering wheel are black inside of a yellow roadsign.
Usually, it is accompanied by the message, "Your driving assisted <player>."
The medal can be achieved easily with any map that has a Warthog, especially in a Big Team Battle match.
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, Oddball or Capture the Flag game.