I think Harry got the answer to his specific question in that other post, the adapters don't cause the speedo to be more accurate than they are without adapting
how our speedos work - yes, permanent magnets do weaken over time [although that'd likely affect the full range], and the hair-springs that create the "balance" with the spinning magnet can also weaken - in addition the mechanisms wear, and collect airborne contaminants that will affect the movement - but I think the main thing is that these cheap mass produced instruments aren't likely to be totally accurate over their full range even when new
Bill
