Had a few speedo issues over the years with mine!
One problem was the path the speedo cable was taking - the bend was too tight which caused binding. Easy fix - re-route
Another problem was the right angle speedo drive on the gearbox. Went through several before getting a decent one from speedograph (now defunct). Although it started on the 4 speed, it transfered easily to the 5 speed - thread here:
https://www.talkmorgan.com/ubbthreads.php/topics/562407/speedo-drive-replacement#Post562407Also cut a larger access hole ot the gearbox tunnel to make it easier to fanny about.
Another set of issues was with the speefo drive gear in the gearbox itself. If I remember I'd lost a couple of teeth and had to replace. Easy job in situ, after I'd cut an access hole on the other side of the gearbox tunnel.
Final issue was realising the gearbox main shaft drive gear was wrong which involved stripping the entire gearbox back end down to replace the gear.
Even now, the speedo over - reads by about 8%
The first thing I'd do is remove the speedo drive from the back of the speedo so it's in the cab and go for a drive. If it spins well, then fingers crossed all OK! Also, see if you can hold it lightly when driving - mine span freely but as soon as I put a tiny bit of resistance on the cable it stopped spinning (the quill was slipping in the angle drive).
Not had an issue with the Speedo itself (although the MGB one did fail, and ended up replacing as plenty of spares on eBay)
