The tire/index load charts have always listed the maximum load capacity for any given load index at 35 psi. That is fine for a tire rated 35psi max, but there are so many tires now that can take up to 51psi which should be able to carry more load, but the specs stop at 35psi not matter what size tire or load index. Why?
Always wondered why the tire Load/Index charts list 35psi max load?
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Calling @CapriRacer
Some load charts do include load ratings for pressures higher than 35 psi for XL tires.
When they don't, it probably has something to do with the fact that auto manufacturers aren't allowed to recommend pressures higher than 35 psi for passenger car tires. Some tire manufacturers probably don't want owners to put on undersized tires that require more than 35 psi to achieve the required load rating. If an XL tire is used that needs 45 psi to hit the proper load rating, the TPMS warning systems don't work as well as a safety feature, and they'll be underinflated if somebody inflates them to the pressures on the door placard. These are probably liability concerns.
When they don't, it probably has something to do with the fact that auto manufacturers aren't allowed to recommend pressures higher than 35 psi for passenger car tires. Some tire manufacturers probably don't want owners to put on undersized tires that require more than 35 psi to achieve the required load rating. If an XL tire is used that needs 45 psi to hit the proper load rating, the TPMS warning systems don't work as well as a safety feature, and they'll be underinflated if somebody inflates them to the pressures on the door placard. These are probably liability concerns.
Because the manufacturer has determined to use that pressure for the tires the truck left the factory with. What happens after that is out of their control.
All P metric tires have the same load tables, regardless of what the max pressure is. For Standard Load (SL) tires the max load occurs at 35 psi, and for Extra Load (XL) tires, it's 41 psi.
HOWEVER, you can increase the speed capability of a P metric tire by increasing the inflation pressure (but not changing the load on the tire).
When this was discovered, the load tables had already been established, so they could not be changed. But they could allow for more inflation pressure for higher speed ratings. If you look, you'll notice that H and higher speed ratings all have at least 44 psi max pressure, while V speed rated and higher all have 51 psi max pressure.
Why those 2 pressures? Because it was the Europeans who discovered this and they were using metric units and chose the round numbers of 3.0 bar (300 kiloPascals) and 3.5 bar (350 kPa) which is 44 and 51 in English units.
Here's the interesting thing. If you design tires with different speed ratings , you can use the same construction except for the cap plies. So technically, the load table is a reasonable reflection of reality!
But there is no regulation saying what the max pressure is supposed to be, only that it has to be larger than the max intended pressure, so S and T rated tires can have 35, 44 or 51 psi, H speed rated tires can have 44 or 51, and V and higher have to have 51 psi - because of the pressures the speed rating tests require. But what a tire manufacturer puts on the sidewall as max pressure is up to the manufacturer - except as outlined above. Many tire manufacturers think that using the higher pressure will make consumers think they are getting something better. (They aren't!)
Oh and it is the tire standardizing organizations who publish the load tables, the tire manufacturers merely republish them. In the US, that is The Tire and Rim Association (TRA). In Europe it's ETRTO (European Tyre and Rim Technical Organization).
And, Yes! The tire standardizing organizations are made up of representatives of the tire manufacturers. But it's those guys who actually publish the load tables - and everything else, such as rim contours!
It's great to have experts like @CapriRacer on this site!! Great response, thank you!
