David,
I have found that the post war Douglases are happiest when allowed to rev. Treat top gear like an overdrive for more pleasant cruising once you have got up to speed. My Dragonfly will regularly nudge 70mph on the flat, and my step daughter's Mk5 is about 3-4 mph faster than that. I bought my Dragonfly in 1972 and was thoroughly disappointed with it's performance, thinking - like you - that 50mph was about it's limit, until advised by a long standing Douglas owner that I should rev it harder. Not convinced, I let him prove the point - which he did!!! Since then, it has been revved unmercifully and seems to thrive on it. The highest speeds I have ever managed on level ground are: 58 in second, 73 in third and 82 in top (indicated) - but you have to be cruel and test the throttle cable to the limit. Of course, riding like this becomes tiring (for the rider), so I usually settle for a cruising speed of 58 - 60mph - anything slower, and you have to play tunes on the gearbox. Incidentally, I have lowered the gearing on my Dragonfly by one tooth on the gearbox - to allow it to rev (more like a Mk3 Sports).
Regards,
Eddie.