I have a factory tach for a 76ish truck that reads higher than what the engine is actually spinning. At about 850 rpm, the tach reads about 2000 rpm. Took it to the local speedo/tach service facility and they said there is nothing that can be done with it. A person needs a dumptruck full of money to afford a new one, so I was wondering if anyone had ever calibrated/fixed one before??