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Six-wheel drive is an all-wheel drive drivetrain configuration of three axles with at least two wheels on each axle capable of being driven simultaneously by the vehicle's engine. Unlike four-wheel drive drivetrains, the configuration is largely confined to heavy-duty off-road and military vehicles, such as all-terrain vehicles, armored vehicles, and prime movers.
When such a vehicle only has six wheels by definition all are driven. When it has ten – with two pairs of ganged "dual" wheels on each rear axle as on a GMC CCKW – all are also driven but the 6×6 designation remains. For most military applications where traction/mobility are considered more important than payload capability, single wheels on each axle are the norm.
Heavy hauler and ballast tractor 6×6s have had a long history as prime movers both in the military , and commercially in logging and heavy equipment hauling both on- and off-road.
Most six-wheel drive vehicles have a forward axle and two at the rear , or three evenly spaced in varying steering configurations. Depending on the vehicle's role, the number of wheels varies between six and ten. Drive may be limited to the rear two axles for on-road use.