Two small markings on every truck tire sidewall, a number and a letter, decide how much weight the tire may carry and how fast it may carry it. They are the load index and the speed symbol, and they appear in every manufacturer’s catalog from Pirelli to Firestone. Most people know the markings exist; few have the full tables in front of them. This reference guide sets out the complete load index scale used for truck tires, the speed symbol table in km/h and mph, how to read single and dual markings, and how to use the tables to cross-check any manufacturer’s claim. It draws on the Firestone data book for the full tables and on the Pirelli and Prometeon range book for examples of how a current range is marked. Pair it with our Yokohama load index guide and the master pressure chart.
How to read a service description
A complete service description has up to three parts: the load index (one number for single fitment, two for single/dual), the speed symbol and, sometimes, a second description in brackets. The Firestone data book uses 315/80R22.5 154/150 M as its example. The size is 315 mm nominal section width, 80 percent aspect ratio, radial construction and a 22.5-inch rim. The load indices are 154 for single fitment and 150 for dual fitment. The speed symbol M means 130 km/h. Together they say: this tire can carry 3,750 kg in single fitment or 3,350 kg per tire in dual fitment, at up to 130 km/h.
The Pirelli range gives more examples. The 315/70R22.5 appears as 156/150 L (154/150 M), the 315/80R22.5 as 158/150 L (156/150 M), the 385/55R22.5 as 160 K (158 L) and the 385/65R22.5 as 164 K (158 L) with the HL (high load) suffix. Read the bracketed part as the alternate rating. The second marking trades load for speed: a slightly lower index at a higher speed symbol.
The load index scale
The table below gives every second load index from 120 to 180, with the maximum load per tire in kilograms and pounds. For the intermediate values, interpolate or consult the Firestone book, which lists every index. The values apply at the tire’s rated speed and at the inflation pressure specified by the manufacturer.
| Load index | Max load per tire (kg) | Max load per tire (lb) |
|---|---|---|
| 120 | 1,400 | 3,086 |
| 122 | 1,500 | 3,307 |
| 124 | 1,600 | 3,527 |
| 126 | 1,700 | 3,748 |
| 128 | 1,800 | 3,968 |
| 130 | 1,900 | 4,189 |
| 132 | 2,000 | 4,409 |
| 134 | 2,120 | 4,674 |
| 136 | 2,240 | 4,938 |
| 138 | 2,360 | 5,203 |
| 140 | 2,500 | 5,512 |
| 142 | 2,650 | 5,842 |
| 144 | 2,800 | 6,173 |
| 146 | 3,000 | 6,614 |
| 148 | 3,150 | 6,945 |
| 150 | 3,350 | 7,385 |
| 152 | 3,550 | 7,826 |
| 154 | 3,750 | 8,267 |
| 156 | 4,000 | 8,818 |
| 158 | 4,250 | 9,370 |
| 160 | 4,500 | 9,921 |
| 162 | 4,750 | 10,472 |
| 164 | 5,000 | 11,023 |
| 166 | 5,300 | 11,684 |
| 168 | 5,600 | 12,346 |
| 170 | 6,000 | 13,228 |
| 172 | 6,300 | 13,889 |
| 174 | 6,700 | 14,771 |
| 176 | 7,100 | 15,653 |
| 178 | 7,500 | 16,535 |
| 180 | 8,000 | 17,637 |
The scale is deliberately irregular. From 120 to 130 the step is 100 kg per two index points (about 6 percent). From 156 to 164 it is 250 kg per two points (about 6 percent). From 170 to 180 it is 300 to 500 kg per two points. The percentage change stays near 6 percent per two points, which makes the scale a rough logarithm of load. A difference of six index points (156 versus 162) means 750 kg or 19 percent more load.
Single versus dual: why two numbers
A tire in dual fitment is paired with a neighbor, which restricts airflow between sidewalls and shares heat. That is why the dual index is lower. For the 156/150, the dual load per tire is 3,350 kg compared with 4,000 kg in single fitment, a derating of 16 percent. For a 154/148, the derating is 16 percent as well. A 148/145 is derated by 8 percent (3,150 to 2,900 kg), because the smaller sizes see less heat. The Firestone book also states that dual capacity is twice the single capacity up to 40 km/h, as covered in our guide to speed and load capacity.
The speed symbol table
The speed symbol is the maximum speed at which the tire can carry the load of its load index. Commercial truck tires are most often F to M. A tire rated L carries its load up to 120 km/h; at 130 km/h it needs a reduced load or a different marking.
| Speed symbol | km/h | mph |
|---|---|---|
| B | 50 | 31 |
| C | 60 | 37 |
| D | 65 | 40 |
| E | 70 | 43 |
| F | 80 | 50 |
| G | 90 | 56 |
| J | 100 | 62 |
| K | 110 | 68 |
| L | 120 | 75 |
| M | 130 | 81 |
| N | 140 | 87 |
| P | 150 | 93 |
| Q | 160 | 99 |
| R | 170 | 106 |
| S | 180 | 112 |
| T | 190 | 118 |
| U | 200 | 124 |
| H | 210 | 130 |
Truck and bus tires rarely exceed M, but the table helps when reading light truck or van tires, which often carry N, P or Q. For tractor-trailers governed to 90 km/h or lower, a G or J symbol is sufficient; for coaches and vans that cruise at 110 km/h or more, K or L is usual. Do not use a lower symbol than the vehicle’s top speed, even if it saves money.
Extra markings you will see
- FRT: identifies tires restricted to non-driven axles, excluding front steering axles. Do not fit them to drive or steer positions.
- HL: high load, a version with a higher load index than the standard tire of the same size.
- M+S and 3PMSF: winter markings. M+S relates to tread pattern, compound or structure and is based on the manufacturer’s statement; 3PMSF means the tire passed a minimum snow performance test.
- Unique point: an additional service description printed near the main one, permitted by ECE Regulation 54. The speed and pressure compensations in the Firestone tables do not apply to it.
- E4 or E-mark: shows ECE approval; the number identifies the country that granted it, where 4 is the Netherlands.
- Regroovable: permitted to be regrooved.
How the EU label relates
Prometeon’s range book also shows the EU tire label grades alongside the load index: rolling resistance, wet grip and noise. A tire such as the 385/65R22.5 164 K with grades A and B and 71 dB conveys three things at once: how much it carries, how fast it may go, and how efficient it is. The label is separate from the load index but is a useful second filter for buyers. Our guide to the EU tire label will follow later in this series, and our existing article on low rolling resistance truck tires covers the efficiency side.
Using the tables in practice
- Check every tire you buy. Compare the load index and speed symbol on the sidewall with the order and the vehicle requirement.
- Never mix indexes on an axle without checking with the manufacturer. If you must, the lower index governs.
- Convert axle weight to load per tire and compare with the table. Aim to use no more than 85 to 90 percent of a tire’s capacity in normal service.
- Check speed. If the vehicle is governed to 100 km/h, a J is enough. If it can exceed 120 km/h, use L or higher.
- Write the index on the fleet specification. Tire specs without load index leave room for substitution mistakes.
- Keep a copy of the tables in the workshop. A laminated page saves guesswork.
Common mistakes
- Assuming a higher load index is always better, ignoring the speed symbol.
- Reading the single index for a dual axle and overstating capacity.
- Confusing the load index with the load range letter (G, H, J). Load range relates to pressure and ply rating; load index is a capacity code.
- Treating the bracketed marking as a typo or ignoring it.
- Fitting FRT tires to drive axles.
- Mixing patterns with different indexes on the same axle.
Cost and risk
A wrong load index is a hidden overload. A fleet that fits 148/145 tires to an axle that needs 152/148 gives away 11 percent of capacity, forcing the tires to run closer to the limit and raising heat and wear. The cost shows in casing loss and early removals, discussed in our article on underinflation damage. Include the index in your tire specification and audit it, and use the total cost of ownership approach to compare the cost of a higher index against the cost of failures.
FAQ
Is load index the same as ply rating?
No. Ply rating or load range describes strength and maximum pressure. Load index is a coded load capacity at the rated speed.
What is the difference between 156/150 and 154/150?
The single-fitment capacity: 4,000 kg versus 3,750 kg per tire, a difference of 250 kg or 6.7 percent. The dual index is the same.
Can I run a tire above its speed symbol?
Only if the manufacturer provides a reduced-load rating for that speed. Otherwise the tire is being run outside its design envelope.
Key takeaways
The load index and speed symbol are the shortest language in tire management. Learn the scale, read both markings, honor the bracket and keep the tables to hand. Then set pressure from real axle weights. Continue with our pressure management hub for the rest of the cluster.
