Did a plug change today old ones had about 25,000 on them and the gap went from .040 to.046.Never could tell any difference stock ngk bike seem to run good and fuel milage stayed the same.
That's exactly what should happen. If there was any noticeable improvement, that means the plugs should have been changed long ago. The same is true with most maintenance items on a GL1800. There are a few exceptions, an example of such would be a valve adjustment.