Most of the a/f gauges are the kind that have the red/ yellow/ green lights or a pointer like on a temp gauge. These are very inaccurite and very hard to read while your driving.They usually come with their own 02 sensor and a fitting that has to be welded into the exhaust pipe. The better choice would be to get a wide band 02 sensor kit. They are much more accurite and read in an actual a/f number on a digital display. They also need an 02 fitting welded into the exhaust. Alot of the wide band kits come with other features such as data logging which is very usefull. You don't have to watch the gauge while you make a banzai pass down the street. You can save the data and read it later. Dyno Jet makes a very good wide band kit. they cost more than a regular a/f gauge but trying to tune with an a/f gauge is a joke compared to a wide band.