P-Tronic additional box-units do not increase the top speed of the vehicle, but they do increase power and torque of its turbo-diesel or turbo-petrol engine. They thus enable the top speed to be reached more quickly, but do not seek to surpass this limit (although it does happen that a vehicle equipped with a box-unit gains a few km/h).

This is explained by the fact that P-Tronic is careful to respect the tolerances set by the manufacturers, and that an increase in speed without modifying the suspension, the brakes, the tyres and other mechanical parts in certain cases could constitute a risk for the vehicle and its occupants.
It should also be noted in this respect that a P-Tronic additional box-unit does not have any influence on the speed limiter fitted to certain vehicles.