If you are thinking of use in fog , they are a waste of time , modern headlights with a very well controlled dip pattern make them unnecessary,
Many vehicles are wired so that you need dipped headlights to get the rear fog to light up, so they end up only being usable as a supplement to head lights in fog , as it is illegal to have them on when vis is over 150mtrs unless they are being used on their own , ie without dipped headlights as you can only have one pair of lights on at a time when dipped. If being used as auxiliary head lights they must go out automatically when dip is selected, unless in reduced viz (150mtrs) . I use mine during daylight as a running lights without dip head on , to save headlight dip filament .