FAR 91.181a states that aircraft on ATS routes shall be flown along the route centerline. I would like a clarification regarding VOR airways. I have a GPS and a VOR receiver, and usually when I fly VOR airways I set both to navigate the leg I'm on. This gives me two navigation needles and two CDI sources. Usually they both line up very well, especially when close to the VORs themselves. A few times when I've gotten far from the station I've noticed a discrepancy between the the VOR nav and the GPS. I assume this is due to inherent error with the VOR as the aircraft gets farther from the station. When this occurs, which navigation source should I use?
I understand VOR airways have 8nm width to account for VOR error, but again, according to FAR 91.181, pilots must fly along the centerline (as best they can anyway). So will the GPS get me closer to the centerline, or the VOR receiver?