Understanding load ratings and speed indices for commercial vehicle tyres is essential for legal compliance and safety. Here's a clear guide for fleet managers.
Reading a Commercial Vehicle Tyre Sidewall
A typical HGV tyre marking such as 315/80R22.5 156/150L contains a wealth of technical information. The 315 is the tyre width in millimetres. The 80 is the aspect ratio — the sidewall height as a percentage of width. R indicates radial construction. 22.5 is the rim diameter in inches. 156 is the single wheel load index and 150 is the dual wheel load index when the tyre is used in a twin configuration. L is the speed index, indicating a maximum speed rating of 120km/h (75mph). Understanding this information is essential for ensuring legal compliance and appropriate tyre specification.
What Load Index Numbers Actually Mean
The load index is a numerical code that corresponds to the maximum load in kilograms that the tyre can carry at its rated inflation pressure. A load index of 156 corresponds to a maximum load of 4,000kg per tyre. A load index of 150 in dual configuration corresponds to 3,350kg per tyre. For an HGV drive axle running twin tyres at maximum axle weight of 11.5 tonnes (11,500kg), the combined load capacity of the four drive tyres must meet or exceed this figure — making the load index calculation a critical compliance check when specifying tyres for any axle.
Speed Ratings for Commercial Vehicles
Common speed index letters for HGV tyres include: L (120km/h / 75mph), M (130km/h / 81mph), and N (140km/h / 87mph). UK motorway speed limits for HGVs are 60mph (96km/h), so an L-rated tyre is technically sufficient for legal UK operation. However, many fleet managers specify M or N-rated tyres to provide headroom and to ensure that tyre speed ratings are not compromised if the vehicle operates across European borders where HGV speed limits may be higher on some road types.
Mixing Tyres of Different Load Ratings
Fitting tyres of different load ratings on the same axle — particularly on twin-wheel drive axles — creates uneven load distribution and wear. On a twin axle configuration, both tyres must carry identical load index and size markings. Mixing even marginally different tyre sizes on a twin configuration results in uneven wear because the tyres have different rolling circumferences, causing one tyre to drag while the other rolls — rapidly destroying both tyres. Always replace twin-axle tyres in matched pairs from the same manufacturer, compound, and production date.
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