MG Car Repair: MGB speedo gears, speedo gears, driven gears

Question
I have a 1968 MGB roadster with overdrive.

The speedo fluctuates a lot but it is not the cable or instrument therefore I suspect the drive or driven gears.

I believe these are not easy to replace but I recently helped remove a 1963 gearbox and the driven gear and pinion came straight out once the housing was removed although the gear was steel.

Will my driven gear be nylon ?

Could this be the problem ?

Can mine be removed as easily as I see MGBhive do a replacement unit for about £30.

I am hoping this might be the case as I don't really want to take out and strip the gearbox

Thank you.

David Croskell

Answer
Hi David.  The speedo drive gear and driven gear are both nylon.  The teeth do tend to strip.  These gears are not inside the gearbox, but in the overdrive unit.  I would extract the driven gear from the overdrive unit and inspect it.

If the driven gear is worn badly, then there is no point in replacing just one part, because the drive gear will be equally as bad if not worse.  That means removing the overdrive and dismantling it.  

If the driven gear is NOT badly worn, then the problem lies elsewhere, and I would suspect the speedo cable has a kink in it.