Wideband O2 sensor ("shoulda bought this years ago")

To the OP, don't be surprised if you don't pick up any MPGs by leaning it out to 15 or 16:1. I use an Innovate MTX-L with a tailpipe clamp so I can tune all five cars. I've found that tuning them lean doesn't help mileage much, if at all, and then you have to put up with mediocre driveability (at least that's what I've noticed, YMMV). I have five cars with varying engine designs. I've noticed that they tend to run the best with 14-15:1 AFRs at cruise. Any leaner and you can feel it, especially on light throttle tip-in. You might find that where you have it set now is where you like it.