bilbo47

Yeah, I think we're on the same page.

Spacers do offset, which pushes the wheel out (towards the outside of the fender)

Centering rings are used to reduce the center bore on the hub; where the wheel sits.
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Most modern cars are "hub centric" and not "lug centric". The wheels sit on the little ring on the hub with almost no tolerance, and then the lugs tighten them down. In older cars, the wheels sits on the lugs while you tighten it down.
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Centering rings are little round hunks of plastic that TireRack throws into the purchase. Or you can buy them aftermarket, and also in Aluminum. People use aluminum because they are afraid they'll melt or deform the plastic at the track.

Factory rims do not require centering rings because (as you'd expect), the factory rim's center bore exactly fits the diameter of the factory hub.

Afaik, most new Audi's use a center bore of 66.56mm. My 18" Enkei rims (for winter) use a 72.6mm center bore. Thus, I needed 72.6 -> 66.56 centering rings.

Go back a bit and you'll find Audi's using a center bore of 57.1mm. To use a stock wheel from a B8 A4 on a B7 A4, a person would need a centering ring from 66.56mm -> 57.1mm
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With respect to offset, and in terms of figuring out if a wheel will fit your car, you really have 3 ways:
1. Find someone running the rim already on your model of car.
2. Find out from the manufacturer. They sometimes list them on their site (like hartmann wheels does)
3. Or do what everyone does. Use the Internet's trusted #1 reference for wheel and tire fitment: TireRack.

Even if you don't buy from them, if the wheel is listed on their site for your make/model, then you know it fits your stock car.
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As for what the OP was asking in his subject line.. I have no idea.

smokeybreeze

When you are looking for wheel fitment for alternate wheels, you really must know the OEM wheel specs *and* the OEM tire size. Why? The accuracy of your speedometer will depend on it, and so may your ability to collect speeding tickets.

So, when you know that, and you find something you want to figure out if it fits, then head on over to this site to see, and play with the different offsets to see what effect that has on your fitment. Tweak other parameters until you really understand what an offset really means or why going from a 245/45/18 tire on an 18x8 +48 wheel to a 285/35/19 tire on a 19x9.5 +35 may work in your situation (it did on my 2003 S8).

http://www.rimsntires.com/specs.jsp

Wheel dimensions are like the laws of physics, backspace is always measured the same way, offset is always measured the same way, likewise for diameter and width. The same holds true for how tires are measured, but with tires there are a couple of different measuring systems (car vs truck, typically).

smokeybreeze

I should also add that TireRack is ultra conservative on fitment, so they severely limit what they tell you fits on your vehicle, but YMMV.

esz

No offense, but are you sure you have the correct wheel size? What year/model is your car? I don't know of an Audi 18" wheel that is 8.5 wide. Hartmann (www.hartmannwheels.com) has some great replicas in stock sizes.

05A632

Yes unfortunately. 2010 A5 with factory 18" wheels. Fairly sure all the A5 factory wheels are 18 x 8.5 actually. Seems aftermarket direct replacements just dont exist.

mssurrey

They are - my 2013 S5 has as stock 18 x 8.5 et29.