Some of these are not the best examples but its what was readily available. I dont have specific years but will try to provide my best guess at a date range. WKC historians may be able to provide a more specific year(s). Here goes:
6" Wheels (starting from upper left to right)Metal hub, rubber tread, thick painted whitewalls = used through at least 1968
Plastic hub, rubber tread, thick painted whitewalls (missing in photo) = used through at least 1973
Plastic hub and tread, thin painted whitewalls = mid to late 1970s
Plastic hub, tread and whitewall = from at least 1979 to current
8" WheelsMetal hub, rubber tread, thick painted whitewall = used through at least 1968
Plastic hub, lawnmower type rubber tread, thick painted whitewall (not pictured) = early to mid 1970s
Plastic hub, tread and whitewall = from at least 1979 to?
Plastic hub, tread and whitewall = from at least ? to ?
Plastic hub, tread with grey cover = from at least ? to ?