i had two stewart warners. an NOS Capri which looked really cool. after greasing the cable, mounting it to be basically shockproof, the inner cable broke the first day. made a new cable for it, and it worked perfect...
...for two days.
then it would just go haywire, and you could hear the cable inside, almost the same sound as the clutch makes on our bikes.
got another SW from a swap meet, and that one worked fine for a few weeks. it was accurate to a few mph, the odometer was almost right on, and then it did the same thing, just spun out.
after taking both of them apart, they look fine, and the needles move when you spin them, but they just don't seem to be able to handle the constant speeds of these bikes.
in my opinion, and from my experience, i'm done with them. at least the bicycle ones. if i can figure out, or find a moped one (that goes over 30) or a motorcycle one, i might give that a try.