FWIW..... if you are interested in economy, then the AFR going into the 15-16 range under light to moderate cruise conditions is fine. (As long as we are not talking about a bog condition....) Keep in mind that under these power conditions, which are well below WOT and max power conditions, the total mixture being combusted is small, and the heat subsequently being generated is low, and thus any danger to the engine is low to none. Keep an eye on engine temps (or better yet, exhaust temps) and if the engine temp stays steady, things are good.
I know everyone wants AFR to stay right at 14.7, but it is not necessary under lighter loads conditions. Just like ignition advances of 50+ degrees under light loads is normal and OK; you would not ever want to do that under max load or WOT, but under light loads, it is fine.
Lean Burn systems would run the AFR's into the 17 and 18 range as a matter of normal operation.
I would expect the AFR meter to be reasonably accurate under cruise RPM conditions and throttle openings. But AFR meters are not always totally reflective of what the real mixture is. In low RPM operation and barely open throttle (light cruise at low speeds) with a moderate cam, the AFR may read falsely rich due to exhaust reversion into the intake. And when cold, and without complete fuel burning, the AFR meter will read falsely lean. So you have to be a bit careful in using the data from an AFR meter; it is all in the nature of them actually being excess O2 sensors, not true AFR meter.
