As far as I've heard, the mechanical speedo is junk.
A bunch of places sell them for around $20, and they're all junk, and they're all the same. The internal parts are all-plastic parts (rotating in plastic bushings) and the sending cable is very cheap and will break after only a few dozen miles (the needle will start to jump up and down when the cable is starting to break).
The last good one was the Whizzer speedo, but not what Whizzer sells now.
The GOOD one was the one from a number of years ago, when they were still being made by Stuart-Warner, which is a real instrument company, and made something that worked well and lasted a long time. Back when you could still get a new one, the speedo head cost like $75 alone, and the complete speedo w/sending cable cost like $130 or so.
Whizzer doesn't use these anymore, they sell something now that is $40 complete, so it is definitely not the same. If it's any better than the $20 junker I don't know.
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