Do your clocks go to 160mph and 70 (7000rpm) ??
As Wayne said the valver clocks will not work correctlly with the VR lump.
The speed sensor is on top of the dif housing on the back of your gearbox, its a hall sender type sensor so they do go faulty from time to time, bit of a pita to get to but a doddle to change, look down the back right hand side of the engine, where the driveshafts meet the gearbox, the sensor is bang in the middle with a 3 pin plug to it.