WhenTurtlesFly wrote:

'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...

U.S. FAR 91.121(a) implies flying by reference to an altimeter whose output
can be adjusted from station readings or actual airport elevation. I doubt
either of your GPS's allow such adjustment settings, so you probably can't
fly VFR cruising altitudes by reference to them.

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

Regulations do not appear to allow setting an altimeter to anything other
than a barometric altimeter setting from a station within 100 NM, other
appropriate station, or to actual elevation of a departure airport.

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

Unless your altimeter is way out of calibration (in which case you probably
shouldn't be flying that airplane until the problem is corrected,) then
everyone else near your aircraft should in theory be flying with similar
offsetting temperature and pressure differences, so long as they are all
"off" in the same way.

(Unfortunately it isn't hard to think up cases where the above fails to
work out.)