Most flight manuals on PA38/PA28 type aircraft list the max pressure error correction as plus or minus 50 ft.
Both the PA-28-161 (REPORT:
VB-1188) and PA-28-181 (REPORT:
VB-790) say:
The altimeter error is less than 50ft unless otherwise placarded
Cannot find anything similar in the few Cessna manuals I have to hand and don't have any grumman manuals. Not sure it helps anyway, all that says is that
in a piper, if it's out by more than 50ft you just stick a placard next to the altimeter and you are good to go.
Well, this is all very interesting. The only
regulation I can find which comes close to this subject are the ones which say your Mode C needs to be within 125ft of what the altimeter reads, and that's only for IFR. It seems to me that there is no rule which says the altimeter needs to be accurate if under VFR, it just needs to be there. So unless somebody can come up with something I think the OP is pretty much out of luck, unless he uses the AIM, which appears to be a dirty word
. One learns something every day, it appears these +/-50ft and +50/-75ft 'rules' are without any regulatory basis. They are just pieces of advice handed from person to person - unless somebody can demonstrate otherwise.