Max load and max pressure

How to read it

FMVSS 139 requires both on each sidewall, and defines them: the maximum permissible inflation pressure is the maximum cold inflation pressure to which a tire may be inflated, and the maximum load rating is the tire’s load rating at that pressure. USTMA’s example reads Max Load 730 kg (1609 lbs) at 300 kPa (44 psi) Max Pressure. For passenger car tires with a maximum of 240, 280, 300, 340 or 350 kPa, the rule requires the psi in brackets after the kPa, rounded up, and the pounds in brackets after the kilograms. The same weight is coded in the Load index (95 in 95H): 730 kg is 97.

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Sidewall pressure and placard pressure

NHTSA’s booklet says tire makers list the maximum permissible pressure on the sidewall because tires are designed for more than one type of vehicle, and that the vehicle manufacturer, not the tire manufacturer, determines the correct pressure for the tires on a vehicle. That figure, the recommended cold inflation pressure, is on the placard FMVSS 110 requires on the driver’s side door pillar, for the front, rear and spare tires. Ford’s manual says the tire’s maximum is normally higher than the vehicle maker’s recommended pressure; Michelin says the MAX figures are not the recommended operating pressure, and Yokohama that the placard pressure rarely matches the sidewall’s.

Higher pressure, not more load

Bridgestone’s manual says passenger tires may be marked with maximum pressures higher than the pressure needed to carry their maximum load, and that the higher pressures add no load capacity. It lists the maximum pressures typically marked: 240, 300 or 350 kPa (35, 44 or 51 psi) on P-metric standard load tires, with the maximum load reached at 240 kPa (35 psi). Ford’s manual says the same of standard load tires above 35 psi (P-metric) and 36 psi (metric).

Light truck tires and 60 psi spares

On LT tires, FMVSS 139 requires the marked maximum pressure to be the pressure for the tire’s maximum load, or up to 69 kPa (10 psi) more at the maker’s option, and LT tires may give separate figures for single and dual use. A tire with a maximum of 420 kPa (60 psi), such as a temporary spare, carries the words “Inflate to 60 psi” or “Inflate to 420 kPa (60 psi)” on both sidewalls in letters 1/2 inch high.

The pressure warning light

The dashboard’s tire pressure warning light measures the other side of this: NHTSA says the monitoring system lights it when tire pressure is below the acceptable level.