Why is it important to know the number of passengers a vehicle will carry?

Understanding the number of passengers a vehicle can carry is crucial for calculating payload. Payload refers to the total weight that a vehicle can safely carry, including passengers and cargo. Each passenger adds weight, which contributes to the overall load on the vehicle. By knowing the passenger capacity, one can accurately determine how much additional cargo weight the vehicle can handle without exceeding its maximum payload limit. This is essential for safety and performance, as exceeding the payload can lead to handling issues, increased tire wear, and potential legal penalties.

The other options do not directly relate to the specific importance of knowing passenger capacity in the context of calculating payload. Knowing the passenger count isn't primarily used for determining fuel economy, assessing cargo space, or deciding on engine size, even though these factors can be influenced indirectly by overall vehicle capacity.