my speedo will be ok up till about 40mph, then it will just start making a whirring type noise and the needle bouces between 0-120mph, sometimes it will just hang down and other times it will say im doing about 75 when i know im only doing 40, and sometimes its fine. i changed the speedo cable and checked the fixing on the gearbox and they are fine, checked the back of the speedo and it looks ok, anychance its the clock thats fucked....maybe the cogs inside or something. i know it aint the cable or gearbox as the milometer works fine. HELP!!!!!!!!!!!!!