The "narrowband" is for OEM applications, and is designed to read from about 13.5 to 15.5 to 1 accurately and reliably. The "wideband" will read from 11.5 or so accurately.
I disagree with this. The narrowband sensor basically can tell you if you're richer or leaner than 14.7. And that's it. The voltage switches so rapidly around the 14.7 point that they're inaccurate more than a few tenths either side.
My Autometer 3579 is not the greatest design (it won't work unless you start the car first, then switch on power to the gauge and sensor) but it is essential for carb tuning. You can also see the change between ethanol-free gas and the typical pump E10 or E15.
I leave it in. About 600 street/cruise miles so far (and quite a few hours idling) and it's still working the same... but my carb is reasonably dialed in (12.5-13 under power, 15 cruise, 13.5-14 idle) and it doesn't have fouling problems.
