A truck tire’s load rating is not a single number. It depends on how fast the vehicle travels and how much air is in the tire. The Firestone 2024 TBR data book, which follows ETRTO rules, includes a pair of tables that show exactly how load capacity changes with speed and with inflation pressure compensation. Most fleets never read them, yet they explain a great deal: why a yard tractor can carry far more per tire than a highway tractor, why bonus loads are restricted, and why a slow-moving trailer should still be treated carefully. This guide explains those tables in plain language. For the foundation, read our truck tire pressure chart by axle load first.
The idea behind the table
A tire generates heat as its sidewalls flex. At low speed the heat is small, so the same tire can safely carry more load. At high speed, heat builds quickly, so the load must be limited. The load index printed on the sidewall (for example 156/150 on a 315/80R22.5) is the capacity at the tire’s rated speed symbol. Firestone’s table 4.2 shows the percentage by which that capacity may change at other speeds, and the extra inflation pressure that goes with it.
The table applies to the speed symbols F (80 km/h, 50 mph), G (90 km/h, 56 mph), J (100 km/h, 62 mph), K (110 km/h, 68 mph), L (120 km/h, 75 mph) and M (130 km/h, 81 mph). Those are the symbols you see on commercial tires, usually after the load index.
How much extra load is allowed at low speed
The table is long, but a short extract shows the pattern. Values are the allowed increase in load capacity compared with the capacity at the rated speed, for tires with speed symbols J to M, plus the inflation pressure compensation in kPa:
| Speed (km/h) | Load increase (%) | Pressure compensation (kPa) |
|---|---|---|
| Static (0) | +150 | +40 |
| 5 | +110 | +40 |
| 10 | +80 | +30 |
| 20 | +50 | +21 |
| 30 | +25 | +13 |
| 40 | +15 | +10 |
| 50 | +12 | +8 |
| 60 | +10 | +6 |
| 70 | +7 | +2 |
| 80 | +4 | 0 |
| 90 | +2 | 0 |
| 95 | +1 | 0 |
| 100 | 0 | 0 |
A few things jump out. At a walking pace of 5 km/h, a tire can carry 110 percent more than its rated load, which is why container terminal tractors, quarry trucks and port equipment can run smaller tires than their loads suggest. At 40 km/h the increase falls to 15 percent. By 100 km/h it is zero: there is no bonus for highway driving. Above the tire’s rated speed, capacity must instead be reduced.
The pressure compensation column
Extra capacity does not come free. The last column shows the extra inflation pressure that must be added to carry the higher load. At 30 km/h the increase of 25 percent comes with 13 kPa (about 2 psi) of extra pressure. At 5 km/h the increase of 110 percent requires 40 kPa (about 6 psi). These values are small, which tells you something about how tires work: the air pressure supports the load only partly, and much of the increase in capacity comes from the lower heat generation at slow speed. The footnote makes it explicit that the increments are to be used in the absence of any specific agreement with the tire manufacturer.
The five notes you must not ignore
- Dual fitment equals twice single up to 40 km/h. At higher speeds a dual’s per-tire rating is lower than a single’s.
- Bonus loads are not permitted at 40 km/h and above if the wheel axles are rigidly fixed to the body of the vehicle.
- Bonus loads cannot be applied to trailers and semi-trailers at speeds over 65 km/h.
- Trailers restricted to speeds below 100 km/h must carry a sign showing the maximum speed.
- The compensations do not apply to the additional service description, known as the unique point.
These notes matter because the table is often used to justify overloading. A fleet with a specialist trailer may argue that “the tire can carry 25 percent more at 30 km/h”. That is only true if the trailer is genuinely limited to low speed, is marked, and the inflation is increased as the table says. The moment the vehicle travels on a public road at 80 km/h, the bonus load is gone.
What this means for highway fleets
For a long-haul tractor-trailer that never drops below 60 km/h on the road, the table has one message: your tire’s rated capacity is your capacity. Speed above the rating reduces it. The ETRTO tables the data book follows allow a reduction in load at speeds above the tire’s symbol, and a manufacturer may specify one. If you run a tire rated 156/150 L (120 km/h) at 130 km/h in a country where speed limits allow it, check with the manufacturer. The same applies to coaches that cruise at higher speeds.
The better question for a highway fleet is whether you are leaving capacity unused. Take a 315/80R22.5 156/150 L. If the load index shows 4,000 kg per tire in single fitment but the axle loads never exceed 3,500 kg, you can reduce pressure to a level that covers the real load. The savings come from comfort, traction and in some cases tread wear, but you must still keep pressure high enough to avoid heat. We explain the method in our Goodyear load and inflation table guide and the Continental table article.
Reading the load index scale
The data book also reproduces the load index scale. The scale is not linear. Each step is a few percent: LI 155 is 3,875 kg, 160 is 4,500 kg, 164 is 5,000 kg, 170 is 6,000 kg, 176 is 7,100 kg and 180 is 8,000 kg. Going from LI 156 (4,000 kg) to LI 160 (4,500 kg) is a 12.5 percent increase. That is why a high-load version of the same size, one or two index points higher, can change what an axle can legally carry.
A practical decision flow
- Identify the tire’s load index and speed symbol from the sidewall.
- Look up the capacity per tire in the load index scale.
- Decide the real road speed. If it exceeds the tire’s symbol, ask the manufacturer.
- Do not use low-speed bonus loads on vehicles that travel above 40 km/h.
- Choose pressure using the load table for the actual axle weight, then add margin.
- Re-check when the duty cycle changes (for example a tractor moved from highway to yard).
Where this leads in cost terms
Operating a tire at the edge of its load and speed envelope shortens its life and reduces casing value. A casing that runs hot is less likely to survive for retreading, which affects your total cost of ownership. Our guide to hot retreading explains why retreaders reject casings with heat damage. And if you are chasing fuel savings, see low rolling resistance truck tires; those tires gain nothing if pressure is wrong.
A practical example: a port tractor and a road tractor
A terminal tractor that moves containers within a port at no more than 25 km/h can use the low-speed allowance in the table, which at 20 to 25 km/h permits an increase in load of roughly 35 to 50 percent, with 17 to 21 kPa of extra pressure. The same chassis on a public road at 80 km/h receives no such bonus beyond about 4 percent, and above 100 km/h none at all. If a yard tractor is later moved to road work without changing its tire loads or pressures, it is silently operating outside its envelope. The lesson for fleets is to document the duty of each vehicle, and to re-rate the tire specification whenever the duty changes. A simple column in the asset register, “maximum operating speed”, is enough to prevent most mistakes.
FAQ
Can I use the low-speed load bonus on a road trailer?
Not at normal road speeds. Bonus loads cannot be applied to trailers and semi-trailers at speeds over 65 km/h, and the bonus does not apply at 40 km/h or above if the axle is rigidly fixed to the body.
Why does a dual tire carry less than half the axle’s single capacity?
Up to 40 km/h a dual’s capacity is twice a single’s. Above that, the dual’s per-tire rating is lower because two tires next to each other share heat and have less room to dissipate it.
What is the unique point?
It is an additional service description printed near the main one, allowing a specific load and speed combination. The speed compensations in the table do not apply to it.
Key takeaways
Load capacity is a function of speed and pressure. At low speed you can carry far more; at highway speed there is no margin. Know your tire’s symbol, respect the restrictions on bonus loads, and use real axle weights to choose pressure. For the full cluster, return to our pressure management hub.
