With this mod the max working speed can be altered on every implement or vehicle. To do so, you just have to select the implement or vehicle.
There is no need to go to the vehicle dealer or a workshop.
The default maximum speed is 42 km/h or mp/h, but this can be changed in the config file GameUserFolder/modSettings/SpeedLimitOverride.xml
The default keybinding (which can be changed of course) is the following:
Left Ctrl-Key + Left Alt-Key + Num + -> Increase limit
Left Ctrl-Key + Left Alt-Key + Num – -> Decrease limit
Left Ctrl-Key + Left Alt-Key + Pos1/Home -> Reset speed to the default value