I installed a digital tach in my car and it's idles at 1000 rpm's and the digital tach it reading 500-580 rpm's. I hooked the the ground, the 12v power source, and ran a wire to the tach wire on the coil. Does anyone know of the reason it is reading low and how can i fix it?

 

Don't hook up the wire to your coil. You will get half the reading. There is a grey/light blue wire behind your gauge cluster. Hook it up there and should read fine.