Never takes as long as you think it should to overtake you does it!
Here is my point of view on this topic.... Our 40K JD's do about 28mph and I will go at that speed when I feel it is safe to do so. I let the type of road, road surface conditions, type of implement or weight of trailer etc dictate what speed I travel at. I am well aware of the law and it is my risk when I break it! When travelling along the A3 from near Wimbledon towards Guildford, for example a, three lane motorway basically I feel it is more dangerous to be going at 20mph when everything else is doing 70mph!