How do car and tyre manufacturers decide tyre pressures?
Proper tire inflation determines the load carrying capacity of a tire. It has nothing whatsoever to do with “deflection” which is a function of sidewall contruction, aspect ratio and speed rating as much as load and inflation. “Deflection” is a meaningless way of determining proper inflation pressures. “Standard load” Passenger Metric tires (American standard) have maximum inflation pressures of 35, 44 or (rarely) 51 psi. 35 psi used to be the norm but many manufacturers have in the last few years increased that limit to 44 psi, usually for tires with higher speed ratings. Standard load P-Metric tires achieve maximum load capacity at 35 psi however so there is no gain in load capacity by increasing the pressure over 35. “Extra-Load (XL) P-Metric tires achieve maximum load capacity at 41 psi which is also typically the maximum inflation pressure although some have a limit of 50 psi. Again this varies by brand and model and there is no load gain over 41 psi. Standard load Euro-Metric pas
