Does anyone know if the wheel off-set is the same on the Audi A4's and A8's?

 

but, the punch line is it will likely work as long as you don't go overboard on widths.

Each wheel has its own offset. Typically for different style wheels for the same Audi model that are the also same width, the offset is then the same. That oversimplifies though where they may still vary the offset for style or other reasons, for example the flared fender cars like the C5 4.2s, the RS4's, etc. And yes, across modern Audi models it usually is pretty interchangable, unless you are dealing with the flared fender cars, or the Q7 or S5 where the bolt pattern and/or center bore also varies.

Also, be careful because an offset is only apples to apples for the same rim width. If you vary the width of a rim on the same car, the offset changes--typically going down as the rim width increases (on a 1 for 2 ratio of offset change to width change). See the link in the post just below about offsets for more info. But even that offset change with width differences may depend on style or clearance related issues.

All that said, owning a D3 A8 (the current body), the typical offset is about 46mm on an 8 1/2 or 9" rim. I don't recall the 8" offset, but it won't be dramatically different. Offsets are also similar for the C6 A6's, and also the C5's that are NOT flared. I think the A4's generally are in the same general offset zone too, though I think of those as well as non 4.2 C5 A6's as using somewhat narrower rims typically that may affect the offset.

If you search the archives (probably the A4 ones and the wheel and tire forum) looking for people who have done A8 fitments, you may find real world experience. I have seen pics here and there of A8 OEM wheels on a variety of other Audi and VW models, presumably because the A8 wheels have the wider widths and bigger diameters to which people want to upgrade.

 

No problems with fit whatsoever.

Not sure about the jump from A4 to A8 though.