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GM never offered Rallys in any lug pattern that were 10" wide from the factory.http://wheelvintiques.com/shopsite_sc/store/html/page38.html
So you are telling me you have seen on the back of one of these wheels 15x10 stamped on it with GM and the date code of 1980? Those pictured are 15x8's.These are the sizes offered for Steel wheels4x2, 16x5 4x4, 16x5 4x2, 15x6 4x4, 15x6, 1/2" offset 4x4, 15x6, 1/8" offset 4x4, 15x8, Rally 4x2, 15x7, Steel Spoke 4x2, 15x8, Rally 4x2, 15x8, Steel Spoke 4x4, 15x8, Steel Spoke 4x2, 15x7, Rally