'Cause I have one- Filed to cruise at 8500. Two GPS's (old simple one in the plane and my new portable) say I'm at 8500, but pressure altimeter says I'm at 8000. I of course set my altimeter to departing airport elevation, which should calibrate it to current pressure and nonstandard temperature, right? Do I...

* fly what seems to be accurate and correct readouts on both of my GPS's? Then risk my altimeter being off and my Mode C reporting incorrectly...

* fly GPS's and adjust altimeter to read the GPS altitude, even though the altimeter setting is not what is reported?

* fly the altimeter set at reported conditions, have my Mode C report accurately, but run the risk of being off VFR altitude?

Thanks