JRB1953

An additional question I have on this is what would cause the factory plugs to burn to .090? I just replaced the factory AC Delco/NGK Platnium PZTR5A 15 with NGK Irridium TR5IX 7397 on my 2003 6.0L GMC 1500 HD at 112,000 miles. I researched and found the factory plugs are gapped at .060. The irridium are gapped at .040. The old plugs were burned to an average gap of .090. The threads on all old plugs had a gold tone on them. The truck didn't run too bad, rough idle every now and then, but fuel mileage sucked. Is this normal or is there something wrong with the old plugs being burned that much? Thanks.