Tire Inflation Pressure Formula Gather Information of the Truck and Tires You going to need some information about the truck and the tires to use this formula. You need to scale the truck as you plan on using it this means with full load of cargo, trailers, full tank of fuel, etc. You need the front and rear axle weights. Then your going to need the information about the tire and its ablities for weight which are typically on the side wall