Adherence to all posted speed limits is required but you must also be in accordance with the top speed rating in the operating manual for your Thule product. However, transporting items of various weights, shapes and sizes alter the way your car behaves in certain situations.
Speed should be adjusted to suit the load being carried, other parameters such as road conditions, wind, traffic intensity and so on, and of course, traffic regulations as well.
With a load on the roof, the vehicle's driving characteristics, its braking performance and its sensitivity to side-winds may change. Care should be taken in all driving conditions to maintain control at a safe speed.