The AFR gauge is one of the things I recommend to most people.  It is argued that an AFR gauge is a waste of money because of it's inaccuracies.  While it may be inaccurate, it is the cheapest way to keep tabs on dangerously lean condition in your engine.  The Cyberdyne/Intellitronix/Nordskog AFR gauge is priced around $30.  It is cheap insurance to install one in your vehicles to keep track of changing AFR conditions while driving.  I do not endorse using these gauges for fine tuning, but I use them for general tuning (fuel computers or stand-alone EMS units), as they are better than nothing at all.

The AFR gauge relies on your stock O2 sensor to monitor combustion gases exiting the exhaust system.  The O2 sensor is typically a narrow-band (0-1VDC) zirconium-based sensor.  This sensor generates it's own voltage ranging up to 1VDC depending on O2 content in the exhaust gas.  As O2 levels go down, the voltage goes up.  This is not a linear signal, so thus the reason why you do not fine tune using the O2 sensor output; the signal is more like an on/off switch - the important crossover being the perfect stoichiometric ratio of 14.7:1.  14.7:1 is the magic ratio of air to fuel, as this gives you the best combustion efficiency without going into dangerously lean condition which can cause detonation.  Anything richer or lesser than 14.7:1 is just wasting fuel and fouling spark plugs.  This is for low-load condition, as high-load/WOT AFR's are richer to make more power and surpress detonation - turbos can run as rich as 11.0:1.

Since the O2 sensor output is a DC voltage, all AFR meters are based on the LM3914 chip.  The LM3914 chip takes a voltage input and spits out 10 separate signals, depending on voltage level.  There are several write-ups on the Internet that offer instructions to build your own AFR gauge if you're able to.  Due to the inaccuracies of the narrow-band O2 sensor, a 10-LED segment meter is good enough.  The Cyberdyne/Intellitronix/Nordskog gauge is the cheapest out of the lot.  Other brands include K&N, Split Second, and EFI Systems, but cost a whole lot more!  Autometer offers a 20-LED meter, and Halmeter offers their AF30 with 30-LED's, but none of these offer better accuracy or resolution than a 10-LED meter.  Some of these cost over $100!

Racers Wholesale - Splitsecond, K&N, and EFI unit

Summit Racing - Nordskog gauge, part number NRD-M7009
Jegs - Cyberdyne gauge, part number 280-7009

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