While I no longer use the Lucas cyclometer, I did in the 70's. I had it mounted on my training bike and never experienced any problems with it. My speeds on club rides often exceeded the maximum that you quote and, of course, I put many miles on it with no failure. It finally disappeared while the bike was in a bike shop for repairs. The mechanics couldn't even remember it being on the bike.
Paul Patzkowsky
Longmont, CO