The dash unit is probably worn out or gummed up.

Take it out and look.

 

Check the cable. It unscrews from the back of the speedo. Pull the core out and see if it's broken. Next, check the gear though there is little chance the speedo would work at all if it has broken teeth. Last, the head unit as it's the most difficult to pull. It might be that the rotating magnetic piece needs help then it's off to the repair shop

 

speedo

pull the speedo out... when the cable spins it spins a round magnet which spins another magnet that moves the indicator. u most likely have a small bind on the indicator side that either a little lub or tweek with a small screw driver to free it up. just be careful, it don't take much to mess it up.

 

sadly i've had the speedo out of my 58 more times than i dare to say
what ever you do... DO NOT buy an aftermarket replacement, i've had 3 'new' pos speedos in my truk

you ''can bump the dash with your palm and the needle jumps up then back down instantly''
this leads me to think there is a screw loose on the head, opening the gap between the magnet
if the needle was hanging on the speedo face plate (common), when you smacked it the needle would jump up and stay at speed

first - check the odometer reading by driving 5 miles at 60 mph and see if the odometer goes 5 miles in 5 min
you can use a speedo app on either android or iphone to check speed and mileage
if it reads right this indicates the cable is engaged at all speeds (may be a simple as a loose speedo cable in back)

this pic is the 55-59 chevy truck speedo head:
the casting has the spinning magnet, on the left is the clock spring, disk and needle, you may find these 2 screws loose



to pull the cluster you need to disconnect the battery first, then speedo cable, oil pressure line and all the wires under the dash
i modified my cluster by converting the ammeter to a volt gauge and added enough wire to pull the cluster out to disconnect



while i had it out i changed the odometer to read actual miles since the frame of build



i ended up rebuilding and recalibrating an old speedo, now it reads correctly

 

While the speedo head is the most likely source of the problem, understand that there is another, more obscure failure mode that can cause this. I've had the plastic speedo drive gear in the trans strip its teeth, due to a binding speedo cable. The speedo would read up to a certain speed, then stop. Turns out that the thick trans oil was providing enough "hydraulic" coupling that the speedo driven gear would turn to a point, until the friction in the cable overcame the hydraulic coupling force.

Since it's relatively easy to check, I'd pull the speedo gear adapter out of the trans and look at both gears before diving into the speedo head. Admittedly it's a less likely cause, but it's a lot less work to check first. You can also access the driven end of the speedo cable and spin it with an electric drill to assess speedo head performance.

 

Thanks for the replies i will give them a looking at.