Tyres are the only point of contact between a heavy goods vehicle and the road, and their construction has a direct bearing on fuel consumption, payload and ride comfort. Two technologies in particular dominate the specification of a modern long-haul tractor: tyres designed to roll with as little resistance as possible, and the light-alloy wheels on which they are mounted.
Rolling resistance is the energy lost as a tyre deforms and recovers shape while it rotates under load. At motorway speed it accounts for a major share of the total energy a tractor has to provide, so it is a primary target for fuel saving on long-distance work.
To address this, manufacturers offer dedicated low-rolling-resistance (LRR) tyre lines, often marketed as "energy" tyres, in which the casing construction, tread pattern and rubber compounds are optimised to minimise the energy lost in each revolution. Long-haul fleets fit these in preference to general-purpose tyres precisely because the reduction in rolling resistance translates directly into lower diesel consumption over high annual mileages. A representative fitment for a long-distance tractor is the Michelin X Line Energy range, with comparable lines from other makers such as the Continental Conti EfficientPro.
Heavy goods tyres are described by a standard size code giving section width, aspect ratio and rim diameter. A widespread size on driven and trailer axles is 315/70 R22.5. Where a wider, lower contact patch is wanted on the steered axle, a low-profile fitment such as 365/55 R22.5 may be used, the lower aspect ratio giving a shorter sidewall and a broader tread for stability.
The wheel is the rigid disc that carries the tyre and bolts to the axle hub. Heavy vehicles have traditionally used steel wheels, but long-haul tractors increasingly run light-alloy wheels made from aluminium, a representative example being those produced by Alcoa.
The chief advantage of an aluminium wheel is a reduction in unsprung mass — the mass of those components that are not carried on the vehicle's springs, principally the wheels, tyres, hubs and brakes, and which move up and down with the road surface rather than being isolated from it by the suspension. Lowering unsprung mass lets the suspension follow an uneven road more closely and keeps the tyre more consistently in contact with it, which improves both ride comfort and roadholding. The relationship between unsprung mass and ride quality is discussed further on the Suspension page.
Beyond comfort, replacing steel with aluminium reduces the overall weight of the vehicle, so a small amount of additional payload becomes available within the same legal weight limit — valuable on operations that regularly load to the maximum. Aluminium also resists corrosion better than painted steel, helping the wheels retain their appearance and integrity over a long service life.