I have found MZ speedometers pretty accurate.

Modern RE ones seem pretty accurate if the Himalayan one is anything to go buy - always slightly over by a couple of MPH, never under.

My old Bullet had an "approximeter" rather than a Speedometer, it was erratic to say the least.

I fitted a wired bicycle speedo on that one and it worked with no issues.

The GP pushbike speedo that I fitted to the Morini was OK, but quite slow to react - the Morini has now gone onto the next owner.