Improving MPG
... You are looking for as close to 14.7:1 mixture. ...
I somewhat disagree with that statement.
14.7 is just a number, a reference point, which shows all the (this type of) fuel is burned perfectly with all the available air.
I doesn't say much about mileage.
When an engine isn't 'working' like when idling or just cruising down a level road, you can tune pretty much as lean as you want as long as it's able to maintain the rpm or speed wanted without having to give it more throttle.
I'm running propane in my daily Dart where the stochiometric value is around 15.5-15.7. But I pretty much always cruise and drive around with the mixture in the 16-17.x AFR range.
At idle the AFR is only at 18 AFR most of the time.