Every new car sold in Australia has a “Tyre Placard’ fixed to the vehicle, generally on the front driver’s side door pillar or door. Information is given there on what tyres are permitted to be fitted to the car, and pressure requirements for varied operating conditions, such as basic, high load, and high speed. Dependent on which model of the car, these may vary. Maybe the luxury version differs from the turbo-charges sporty version- at least you would hope so!
The Australian Design Rules compel manufacturers to fit tyres which match or exceed the performance of the car under various load and speeds. The tyres sizes fitted today have more than adequate load carrying capacity. Fashion has taken care of that, as car buyers like to see the wheel well full of wheel equipment, not the mechanical underpinnings.
From the mass of the car (see handbook), split it into four, consult the Tyre and Rim Standards, and it will reveal that the placarded tyre pressure comfortably exceeds those specified by the tyre makers, if load was the only consideration. Vary the speed, add in the lack of maintenance of tyres generally, and the need for a buffer over the minimum pressures specified is apparent.
The car engineers also want to get the ride and handling package right for the car buyers. The tyre is contributing to the total suspension package, and adjustment to pressures contributes. Higher pressures give better fuel consumption too! There is also the question of reliability and durability to be considered.
The placarded pressures are set for the fully fuelled vehicle with two in a two seater, three occupants each 68 kg in a five seater. Higher load means for five occupants and their luggage, which weighs 13.6 kg for each seat. The weight of passenger and luggage are defined in the Standards. These increased loads are compensated for by higher pressure.
The speed factor cannot be ignored. A higher speed rating on a tyre wall implies that you can go faster, with appropriate pressure increases. But the law won’t allow it is the cry from all but the Northern Territory. What it really means to the motorist is that the premium tyre has design features which resist degradation due to heat, and deformation due to speed or overload. It holds together better under duress.
The Tyre Standards represent the accumulated wisdom of car and tyre engineers over the years, and should not be ignored. When in doubt, or if the car is going to be used under more severe conditions than the basic levels outlined, tend towards the higher end of the pressure ranges specified on the placard.