Tyre load index and speed symbol calculator: kg per tyre and per axle

Reading the service description

A truck tyre’s sidewall carries its size followed by a service description, for example 315/80 R22.5 156/150 L. The first number is the load index for a single tyre, the second the load index for each tyre of a dual pair, and the letter the speed symbol. Continental’s data book explains that UN Regulation 54 requires tyres for road use above 80 km/h to carry this designation, load index (single/dual) and speed symbol. For 156/150 L:

• load index 156: 4,000 kg on a single tyre, 8,000 kg on an axle with two single tyres;
• load index 150: 3,350 kg on each tyre in dual fitment, 13,400 kg on an axle with four tyres;
• speed symbol L: 120 km/h.

The axle figures are this site’s arithmetic (two or four times the load per tyre). A tyre may also carry a second service description in brackets, such as (154/150 M): a lower load at a higher speed.

The load applies at the speed of the symbol

Continental’s data book defines the load index load as the nominal load capacity, 100 %, and the speed symbol as the maximum speed for that nominal load. A vehicle built for a lower maximum speed may carry more, and one faster than the symbol less, according to the load and inflation tables in the data book; the calculator gives the nominal load only. The same book says that trailers must have tyres for at least 100 km/h unless the trailer is clearly marked for a lower speed.

Which load index an axle needs

Divide the axle load by the number of tyres on the axle and look up the lowest load index that carries that much: a 7,500 kg axle on two single tyres needs 3,750 kg per tyre, load index 154; an 11,500 kg axle on four tyres in dual needs 2,875 kg per tyre, load index 145 (2,900 kg). This is the arithmetic of the table, not a fitment rule: the vehicle maker’s documents and the tyre maker’s tables decide the tyre for a vehicle.

Where the table comes from

The load index table runs from 60 (250 kg) to 200 (14,000 kg). It is the table the Irish Road Safety Authority’s CVRT testers’ manuals use, “extracted from UNECE Regulation 54”, and it is identical, entry for entry, to the table in Continental’s truck tyre data book (our comparison). The speed symbols F and G (80 and 90 km/h) are from that data book and J to Y from Continental’s speed rating page.