In article ,
says...

'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?

Dunno why you ask. It's simple. Just pretend you have no GPS. In
short, if you are VFR, you fly according to the reported local area QNH.

The consequences, I guess, are that you will not otherwise be correctly
relative to other VFR traffic.

--
Duncan.