Yup, it was the cable. Symptom was squeaky noise, in sync with road speed, but not related to brakes, etc. First I tightened the lug nuts!
It was pretty easy to lube. Look under the dash up at the speedo. Get a good visual so you can put it back right. I had to remove the heater/AC control panel (3 screws) to get access There is a clip on the back of the speedo. Push it in, hold it in, pull out the cable housing. Snake it around to where you can get at it easy. Look in the end and remember how deep the cable end is. Reach in with some needle nose pliars and pull out the cable. I don't think mine has been lubed in 30 years/300,000 miles! It was a bit rusty near the top, and gummy further down. Slide the cable back in while greasing it. I used my favorite white lithium base grease. At the end twist a bit while pushing in so it goes in as far as it was before. Then snake the cable back where it was, push it back in the speedo until the clip clicks. Done!
now: '73 LT1; '79 Z28
then: '67 coupe; '88 IROC; '95 Z28