6x4

The notation 6x4 is a wheel-drive formula used chiefly for trucks and other commercial vehicles. The first figure states the total number of wheel positions on the vehicle, and the second states how many of those are driven by the engine. A 6x4 therefore has six wheel positions, four of which receive drive, while the remaining two simply roll and usually do the steering. In this counting convention a pair of wheels on a single hub, as found on twin-tyred drive axles, is treated as one wheel position.

In practice a 6x4 is almost always a three-axle truck or tractor unit with one steering axle at the front and two driven axles at the rear, an arrangement often called a tandem drive. The engine sends torque through the gearbox to the foremost rear axle, and a through-drive or inter-axle differential passes power on to the rearmost axle as well, so both rear axles pull. The front axle carries the steering and a share of the load but is not driven.

This layout is the workhorse standard for heavy haulage on sealed roads. Two driven rear axles spread the weight of the load and the drivetrain over more tyres, which both protects road surfaces and meets legal axle-weight limits, while delivering enough traction to move very heavy gross combinations on tarmac. It is the typical configuration for long-distance articulated tractor units, tippers, mixers and rigid trucks that spend most of their lives on the motorway network.

The formula is best understood by contrast with its neighbours. A 6x2 also has three axles but drives only one of the rear pair, the other being a non-driven supporting or lifting axle; this saves weight and fuel for lighter or part-loaded work at the cost of traction. A 6x6, by contrast, drives all three axles including the front, trading economy for maximum grip off-road. The 6x4 sits between them, prioritising load capacity and on-road traction over the extreme capability of all-wheel drive.

There are trade-offs to the tandem-drive arrangement. The second driven axle, its differential and the inter-axle linkage add weight, mechanical complexity, rolling resistance and cost compared with a 6x2, which is why operators choose between them according to terrain, payload and fuel priorities. On loose or slippery surfaces a 6x4 still has no driven front wheels, so it can be outperformed by a 6x6 where conditions are poor.

The drive-formula notation extends across the whole range of vehicle layouts, from the familiar 4x2 of an ordinary two-wheel-drive car or light truck through 4x4 to the heavy 8x4 and beyond. Understanding 6x4 alongside related terms such as 6x6, four-wheel drive and the basic concept of a driven axle gives a clear picture of how manufacturers match traction and load-carrying ability to a vehicle's intended task.