If I do a relatively long East/West leg (yesterday was Biggin to Exeter, for example) if find that the line on SD is significantly displaced from that on the plumbed in GPSs (530/430/Pilot IIIc) to the extent that there is probably over a mile between them at the mid point. Thus, one can have a significant fly right on one and fly left on the other.
Is this because the GPSs use GC and SD rhumb?
I think that the standard is GC. This could cause an issue tracking an airway centreline, I guess.