I have an 80 z28, all original 52k miles. The tach stays at about 5000 rpm,even when not running.It will climb to 7000 when revving. The gas guage reads between 1/4 and 1/2. Just filled the tank, no change.Any help would be greatly appreciated, thanks, Grigs

 

I'd start with pulling the connector to the instrument panel and carefully cleaning the terminals. Then plug it back in to see if that fixes it. It sounds like a resistance or connection problem.

 

hate to say this but your tach is probably shot. when i finally got the tach connected in the 70Z it didn't work right. the needle was off pretty bad. bought a rebuilt one and put it in and it works flawless. I've found that you can't really mess with the dials and get them working again.

and for the fuel gauge, you should be able to check the ohms from the sending unit under the dash. there is a body panel connector under the dash near the fuse box that has the wire for the sending unit. find a wiring diagram for the car. you can test the ohms from there. i think with most GM cars, the full reading was 90 ohms and empty was 0 ohms. truthfully, i'm thinking you need a sending unit for the tank. usually the fuel gauge rarely goes.