>>>I met a Mustang II flyer that has fuel/air ratio gauges for sale...

Lorn,

Eventually the lead in the avgas will "poison" the sensor and it will cease
to function correctly until you get a new sensor.

I would only recommend its use if you are running unleaded automotive fuel
or for very short periods while performing carburetor/fuel injection
diagnostic work. I have used them for years and they are invaluable for
sorting out carb jetting and transient fuel problems.

Regards,
Mike Hutchins