A mosquito's flight speed is relative to its surrounding environment, making it complex to determine its speed in relation to a moving vehicle. While the car travels at 90 km/h, the mosquito appears to be flying steadily in the air inside the car, but its speed is not 80 km/h relative to passengers. Instead, it is moving at a different speed relative to the air outside the car, which is also moving at 80 km/h. The discussion highlights that speed must be defined in relation to another object, emphasizing the mosquito's flight dynamics in relation to air resistance. Overall, the mosquito's actual speed through the air is likely much slower than it appears when considering the movement of the car.