15x6 backspacing, 80's F-150's, vans, etc..

anyone know the backspacing for a 15x6 5x5.5 pattern basic steel wheel? I cant find it anywhere online.
Would it have varied through the years and from vehicle to vehicle? I guess
that lug pattern fit trucks and vans from the 50's to late 80's, some 80's lincolns and possibly other full size.
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