Sounds like your speedometer is either worn out or most likely, gummed up. I've heard of people just removing the cable and spraying some kind of lube into the back of the speedometer. That sounds risky to me so I would remove it, see what I could clean up and find something to lube it that would not splatter all over the inside of the instrument cluster. You should be able to tell from the residue what was originally used for lube.
Easy to get to couple of screws to undo the dash but be VERY CAREFul
once you get to it.I held mine down with a tooth pick lightly after
using a hairdryer to warm it up to soften it,then pulled the tip up
with
a thread while still holding it down mid way with the tooth pick.I
didn't want to take a chance of getting it off the pin.
I had this same problem about 10 years ago in my 1990. It turned out that the needle was scraping on the back. Noticed that it was making a slight line on the backing (with the numbers on it). Tried exactly what gogiburn said. However, I still broke the needle. But I thought, what have I got to loose. Pulled the needle off the pin, glued it back together, with a small sliver of toothpick on the back (it's 'V' shaped anyway). Then went down the freeway with my wife beside going exactly 60 mph and stuck the needle back on indicating 60. It's been that way for the last 10 years. No problem.
my HB had the same problem. it was very very hard to see the bent needle. it catchs on the very very tip and it doesn't drag all the time it seems to float up & down, when down it stops. i couldn't buy a new needle the speedo shop installed an old one.
Use steam to clean it...I had the same problem on a Mustang I had but all speedos are pretty much the same I have been inside the one on my HB enough to tell ya. They put a fatory grease of some sort on the gear you will have to use the steam to spray it off then apply a TINY TINY TINY, did I mention TINY amount of white lithium grease to the gears. Should work well after that. If not then remove the speedo briskly walk to the garbage and give it a good toss, then get you a new one.