Speedometer replacement

This is from last summer. Right around my birthday, I bought a used speedometer head from the guys at BTDT Racing, a local Spec Miata outfit that also sells used Miata parts. Why? My speedometer did weird things in the winter.

It makes this chattering/chugging/scratching noise. It's weird. While it's doing this, the needle will seriously bounce across a range of 15-20 mph until the car warms up, and is exacerbated by bumps even when it is warm. I learned to tell speed by my RPMs.

The noise will get louder, turning into a constant whooshing kind of sound, while the needle shoots WAAAAY up. Seriously, I saw 120 mph indicated when I was going about 65. Thankfully, I clocked this and it did not affect the odometer, just the instant indicated speed. I won't bore you with the details, because this is boring enough.

I tried lubricating the speedometer cable many times with a penetrating graphite-based lubricant -- the proper kind to use on cables and such -- to no avail. So I just got a new head. To test it, I did what you see above: just stuck the head onto the cable and took it for a spin. It's really disconcerting to drive a car without most of an instrument cluster.

Anyway,the needle in the new cluster was still just a little bouncy, but much better than before. The real test was coming this winter.

Which is when I found out that it's considerably better for accuracy and doesn't do the shooting-way-up thing it did before, but it's still really bouncy when cold and makes that scratching noise (actually, Eric likened it to the chugging of a steam engine). So I included a new speedometer cable with my recent order of engine parts from Mazda Motorsports. In the mean time, I did get improved functionality. 

And my trip odometer had stopped resetting the tenths place on the old unit. Don't worry: I was honest and replaced only the trip odometer reels and the speedometer head. I kept my original 229k-mile odometer in the car. Below is the final result. I'm currently reading more than 240k miles, but my trip odometer works and the speedometer works better (though not perfectly). I'll put in the new cable soon; I'm certain now that's the primary culprit.

Update: Fixed!