My '83 5000TD didn't come with a tire iron, so I pulled out my trusty 'universal' tire iron to pull a wheel for inspection. Well, the normal VW/Audi 17mm size ALMOST fit, but it was a bit LOOSE.

So I tried a few open end wrenches on the wheel bolt, and it is bigger than 16mm, but a little smaller than 17mm. Huh? They appear to be the factory original bolts, and they all match.

I haven't had any free time to try this yet, but are they possibly an English size? Anyone with an Audi this old have any BTDT?