A/F/R gauge for tuning
I have an AEM mounted in my van, along with a vacuum gauge. Immensely helpful being able to monitor what is going on at every throttle position, especially if there is some sort of circuit issue, transitions etc. Allows you to zero in on things rapidly. I never was good at checking plugs which I attribute to laziness.
I can rely on those two gauges along with feel, listening, seat of the pants and have gotten pretty good at dialing what I have in. It’s quicker than pulling the doghouse and checking plugs in my case that’s for sure. And then at the track I just use MPH as the other gauge.