The G5 EFIS that I fitted to my FX-3 initially read 40 ft higher altitude than the GSU 25 altitude displayed on the GDU 465. After about 2 years the difference had increased to 60 ft and all the error seems to be in the G5.

I mentioned this to a friend who works on multiple aircraft and he commented that he had seen other G5 with poor altitude calibration.

If you have a G5 installed, what difference do you see between GDU and G5 when both have the same baro setting?

The 60 ft error bugs me but there is no way to enter a correction offset. Baro settings cannot be split when the G5 is fully integrated with G3X. The G5 must be calibrated at sea level, 10,000 ft, 20,000 ft, and (optionally) at 30,000 ft. That requires paying someone with an approved air data test set or designing and building my own test set. (That project is underway and I'll report if it works.)