Tire load and inflation tables are based on various standards provided by tire authorities like The Tire and Rim Association (TRA), The European Tyre and Rim Technical Organization (ETRTO) and The Japan Automobile Tyre Manufacturers Association (JATMA). There are four main tire size standards – Metric, P-Metric, LT-Metric and Flotation.

See:
Metric Tire Load and Inflation Table
P-Metric Tire Load and Inflation Table