(See the VDO articles for later Speedo problems and/or calibration)

Here may be your problems, in an order of likelihood;

1. The cable is "binding" at its clip point on the speedo. Some mildly frustrating fiddling can cure that problem..it is related to the length of the cable tip entering the speedo. I use the reversible drill to check functioning here and after it works smoothly with the drill I clip it on the gearbox.

2. Bad routing.  Here is a picture of the original cable that graced the cars..and ALL Plus 8s from the introduction of the 5-speed in 1977 until the advent of the 3.9 dictated the need for a transducer. It sells for $30..where the later ones with a transducer have three parts (lower cable/transducer/upper cable) for a total of $170 IF you can find one. The Morgan cable is 53 inches long and a bad routing can create binding.

Morgan had two speedo routings. One went out the side of the bell housing and then back in to the car through the firewall. Even at 53 inches this stretches the cable so tight that the cable will inevitably (5 minutes - 20 years) bind or snap the plastic press fit lock. The other routing...which is less "clean" but safer, has the cable coming out a hole (through a rubber grommet) in the top of the gearbox cover and then under the padded leather cover until it ends and then loops back into the speedo. This routing leaves more slack and less chance of binding.

3.  Dirty or worn cable sleeve. The cable sleeve is dirty, defective or has oil in it. It can sometimes be rectified by running the cable through it a few times to clean it. With the cable sleeve unclipped/removed at either end (nor necessary to do both) it is a simple matter to remove the cable from its sleeve, clean it, lubricate or replace it. I maintain mine once every year as I find it gives me a steady needle on the speedo. Lubricate with speedo cable lubricant or powdered graphite.
 
4.. The ends of the cable have worn. This problem normally means that the female entry point has worn as well. A worn piece wears everything it touches.

5. The speedo gear has gone. These little plastic gears are very cheap and, of course, no one who has them will supply them to you. This is another area where I would like to get a supply source up on GoMoG. The gear costs pennies, is easy to replace and saves a $125 USD or 90 pound service charge and two week turn-around time.

6. The speedo needle is stuck. On some speedos, for some bizarre reason, the needle can go too far back to it is beyond zero and then it can jam itself at 0. This is easily remedied.  I just fixed the same problem on the e-type with its Smiths.

7. The gear box connection is askew. When you get everything together, be very careful with the connection at the gearbox...you must have it perfectly placed and tight or the cable will not like you either. If the small bracket at the gearbox entry is loose, it will give you a very nervous needle..with a sad up and down movement straddling 5 to 15 mph.

8. Later cars have a speedo cable in three parts, a gearbox to transducer section, the transducer and the transducer to speedo.  The parts can be purchased in sections but are becoming difficult to find. You can have one made. see http://www.speedycables.com/Control_Cables.htm The transducer sends a signal necessary for the smooth running of the engine to the ECU. If this section is broken or the transducer is fault you will have a red warning light and poor performance. Often, the transducer ground is at fault. Simply verify that and off you go.