When traveling at high speed, there is a chance of generating too much power and overloading the power regulator. However, the power generated is related to the revolutions of the wheel together with the rated output of the hub and not the exact speed. For a 20" wheel, sustaining speeds over 25km/h (~15.5mph) for long periods may overload the power regulator so we discourage riding above this speed with the power regulator engaged. 3W hubs can also generate more power than 2.4W ones under the same speed. The 20"-25km/h rating is for a 2.4W hub.
For larger wheels, proportionately greater speeds can be sustained. Our tests show that a standard 26" wheel can go up to 35~40km/hr (21~25mph) without any problem for the power regulator.