hi, it sounds very much like the problem is the speedo, it would be very unlikely that the problem is in the gear box. i would suggest trying to get hold of a second hand speedo from a scrap yard and try that, the only other option is to try and take your existing speedo out and flush it with wd 40 or something simular to try and lubricate where ever the problem is, but be warned even if this works the speedo might give you an incorrect reading, the best way to try and check if it is roughly working is to either find one of those speed warning signs or follow a friend in another car and get them to signal when they get to 30 and then 60 mph, but this will only give a rough guide, as a rule speedos should be chucked if they are faulty or sent to a specialist for repair. good luck, steve i am always in shit, its just the depth that varies