Hi everyone, I'm looking for some advice on a change I plan to make to the wheels of my S5.

I've currently got the stock 19" alloys but really like the look of the 20" RS6 alloys. I've been told they are a straight swop but worried about the following:

Speedometer reading accuracy

Tracking and suspension setting

Loss of acceleration and or top speed

If anyone has any experience of this id really appreciate your advice

You'll need to get tracking redone. What's the width? 

Tyre size will be 255/35/20

Hello Alan,

The simple answer lies with ensuring the outside diameter of the wheel and tyre remains the same as your current 19 inch set up. 
Obviously to achieve this, the tyre profile will need to decrease as the wheel diameter increases. 
You can either use on-line ( I think) calculators, or work it out yourself.

If your current profile is say 40:-

Calculate 40% of the current tyre width, and double it. Then add  19inches converted to mm ( 19 x 2.54). This gives you the overall diameter of the wheel and tyre. 
Do the same for your proposed 20inch wheel and the fitted tyre size, and compare. 

Points that probably go without saying:- ride quality will be adversely affected by decreasing the tyre profile. Your insurers will need to be informed if the new set up deviates from an Audi option when new, or is you are fitting ‘aftermarket’ wheels. 

Kind regards,

Gareth.

Thanks for this, I found and online calculator so will give that a go. They OEM audi wheels I've got my eye on

Coward! Why not do the maths!? 
Only joking. 
Kind regards,

Gareth.