High Idle, Low Tach Reading

So I'm looking for some insight into my issue of a high idle and a low tach reading. Right now my 3.4 auto (99 4runner but same set up as Tacoma) starts at a normal idle, but after a short drive the RPM's jump up high (maybe 1600-2000). Only problem is the tach reads lower than the actual RPM's. It will often read around 900 RPM, but the engine is running around 1600-2000. Also it will read at 0 RPM when sitting in gear. Just took a drive and it was 500 RPM when I was cruising at about 40 MPH. Anyone have or had this issue?

I’ve cleaned the throttle body, IAC and MAF tonight. Friend mentioned the crankshaft position sensor.

 

@mechanicjon

 

I'd have to do a little research on the electrical circuit gor tach. I'm outta town and not near my PC.
Wont be home until Tuesday.

 

Just a guess. Clean engine grounds. Sounds like tach itself is failing. If you have digital odometer,
check the accuracy. Maybe the circuit board is failing?

 

The tach gets its signal from the ignition igniter. The book says check for proper voltage to tach. If it has volts and is not accurate change tach.