Older speedos had an adjustable connecting point for the needle return spring. Just move that till you get a proper reading.
Of course, the hard part is getting the speedo head apart. There is a gauge rebuilding company on the west coast, which does older Triumph speedos and tachs. They might be able to help with this.
If a guy had an "old" speedo to play with, I'll be he could determine where to drill an access hole so he could get at the needle return spring anchor. Then, using a dental pick or o-ring pick, move that anchor point to give a proper reading.
To finish, install a small, black rubber plug in the just-drilled hole.
Do older Triumph models use somewhat the same speedo? A fellow in a "big city", where there's bound to be a motorcycle junkyard, could get one to play with. Once he figured it out, he could do this mod for others (and maybe do ignition box rev limiter mods, too).
Greybeard, Dinqua, Grump, Sal Maglie, Arsenalfan, I hear destiny calling one of you....
