I second what jamie said.

i refer to my AF guage as a dummy light. If i am having probs, i can see if its a major fuel mix issue fairly quickly and decently accurate.

your avg af is useless, just pretty anoying lights. My ears are just as acurate as my dinky af guage. some af guages, wont even read as reach as is needed with some boosted cars.

best route: wideband o2, and EGT guage. Together you can cross refrence them with each other to really see how your cars acting.

usless? imho, no. its a good retard light, tells u quickly if ur O2 sensor is working half way decent, tells u quickly if you have a major fuel problem. ( but then agian, so does a wideband and egt, but thats not the point)