The tach in my 70 Challenger went from fine to possessed yesterday. About 20 miles into the drive it started reading anywhere from 4300 to 5000 rpm and jumping around. It is a new replacement tach bought from Classic Industries Which i replaced the stock tach with during the car rebuild last winter. Here is the weird part, it was working fine and when it started acting up I figured as long as the engine was running fine I would look into it when i got home. i turned the radio on and now its reading as it should around 2800. i shut the radio off and its back to the high readings again. turned radio back on, working. anyone else ever experienced this ? I have not looked at the wiring diagram yet to see if the tach feeds from the same fuse as the radio. not the weirdest problem i have ever had but it is right up there. any thoughts.

That is a strange one! I have not seen or heard of that since Christine. I would confirm it is grounded well for a start & wait for some of the electronic gurus to chime in. & that the 12V feed to it is a secure connection.

Yep, 9 times out of 10 bizarre electrical behavior is caused by bad grounds. Of course there could be a bad ground inside the new tach.