hmm the speedo does run off the gearbox, but it takes a mechanical feed - perhaps something has come loose and is floating round jamming things up? i would have thought gearbox off is an engine up job, as doing the clutch is

probably bad differential box bearing(s) - oil loss and speedo problems, strange noises too. can be checked below the car by trying to shake right inner CV joint up and down...if yes, you need to replace the gearbox...

Thanks for the info. Great help.