This can sometimes be a real problem of a job to sort – so a friend of mine tells me – and he’s bit alone from all accounts.
I found this article on the MG Experience forum which has a vast amount of info – it may help any of us with a similar problem.
It looks like there are different gears for different age of car so I think might be worth checking that some previous owner hasn’t changed the gearbox and or engine to a later or earlier version.
Read the full article here …… https://www.mgexp.com/forum/mgb-and-gt-forum.1/speedometer-drive-gear.3255307/
MGB tips mods and maintenance 