Something that is not well known or understood is the load index of a tyre and how it works. Every vehicle has a minimum tyre load rating which is designed and set by the vehicle manufacturer for maximum performance. This is located on the tyre placard that is usually located inside the door opening on your vehicle. If you have difficulty finding this it is also listed in the owner’s manual.
The load index refers to the maximum load carrying capacity of a tyre at its maximum pressure. This load rating (or carrying capacity) reduces as the tyre inflation pressure reduces. Say the load index recommended for your vehicle is 102, this equates to 850kg carrying capacity per tyre, at the maximum pressure of 44 psi which can also be found stamped on the tyre sidewall.
With your 4 tyres inflated to 25 psi, the load capacity of each tyre is reduced to only 63% of the maximum load rating, and therefore only designed to carry 535kg per tyre. With the 4 tyres that’s a total carrying capacity of 2140kg. On a fully loaded SUV, say a Toyota Rav 4, the gross vehicle mass is 2150kg, therefore the tyres are now overloaded. With the tyres now being overloaded, this may cause overheating of the tyres, poor vehicle handling and potential tyre failure. In any event overloading will reduce tyre life.
This shows the importance of understanding why tyre pressures should be regularly checked and should be increased when a vehicle is loaded. It is also very important to ensure the correct load rated tyre is fitted to your vehicle.