should be 6 x 5.5
Bolt pattern for a 1985 GMC Sierra Classic 1500 is 6x5.5 (That is what mine is)
6x137.5
6x5.5
8x6.5
6x5.5
5x5
Bolt Pattern: 5-114
6x5.5
Service manual
4x114.3 that's the bolt pattern
GMC has three different bolt patterns: The Acadia has a 6X132 the Envoy has 6X135 and the full size such as Yukon and Sierra which have 6X139. Hope this helps you.
6 x 5.5