I have a 89 305. I just bought it a couple of months ago. For some reason the tach reads an idle at about 3500 rpm's. When it is first started in the morning it idles at obout 4500 and slowly moves its way down as it warms up. and then during normal driving with only slight acceleration it tops out. It really does not sound like the motor is revving that high. I would think it would be very loud. I'm guessing it is the tach itself. All other gauges seem to work fine. Anyone ever hear of something like this?

 

Check the connection to the distributor. It should be right in the back, right under cap. You can feel it with your fingers.