OEM 13x5.5 steel and alloy wheels first came on the 1980 Jetta I. The 13x6 steel wheels were only available in Europe, and I've only seen one set mounted on a US stealthmobile, and never found a single one at the boneyard. They're rarer than hens teeth and not worth looking for, IMHO.

I worked hard to get the 14x6" wheels allowed for the ITB Scirocco II, and think they work better than the 13s at most tracks. I'd say that they're faster at Lime Rock and Watkins Glen, although 13s gave me better times at NHIS (probably a gearing issue). The stock 14x6s are also cheap and plentiful.