The "tow rating" in manufacturer's guidelines assumes that your truck or vehicle is unloaded. To calculate a more accurate trailer load, you will first need to collect some real data, including manufacturer-supplied data. Ensure that all of your numbers are expressed in a consistent measurement form, regardless of whether you prefer pounds or kilograms.
Find the booklet provided by the tow vehicle manufacturer. Look up the Gross Combination Weight Rating (GWCR).
Calculate the total actual weight of your vehicle once it is loaded with cargo and fuel. Do this by adding up the weight of all your cargo, including the weight of the people inside. Add that number to the weight of the vehicle supplied by the manufacturer.
Subtract the GWCR from the actual weight. This is your allowable weight.
Multiply by any number from .1 to .5, depending on what margin of error you want to leave. Multiplying by .01 will give you a 1-percent margin of error. Multiplying by .5 will give you a 50-percent margin of error. At least a 20-percent margin is recommended.
Subtract the margin of error from the allowable weight. This is your trailer load.
