HELP!!!! My tach is reading too high

I've tried 3 different tacks, and made sure they are on the v8 setting. I have a mild built 318. It idles in park about 900rpms. When I step on the gas (not redlining it) the tach jumps up to over 8000rpms (maxing it out and almost coming back to the begging). All three tachs were the same. I even swaped out the coil. I have the green wire going to the negative on the coil, black to ground and red to ignition. I even tried to put it straight to the battery. I don't know what to do.