Recently I decided to do a staggered 20 inch setup on my 2006 A6 4.2 After searching the forum and emailing some guys here with 20's, I realized that offsets for 20 inch wheels are a tightly held secret. Most inquiries end with a referral to certain wheel manufacturers that possess these magic numbers.
So for those who need this info here it is. My OE wheel is 17x7.5 with +45 offset, tire 245/45/17
My 20 inch setup
RD Sport (Racing Dynamics) RS-5
Front 20x9 +45 offset, 2" standard lip, 245/35/20 tire
The rear is a little tricky. All the additional room is on the inside.
Rear 20x10 +58 offset, 3" standard lip, 255/35/20 tire- these are Yokohama S drive which run pretty wide. A narrow/rounded 275/30/20 could also fit.
This setup fills both fender wells with zero rubbing, even under maximal load.
My car is lowered with H&R springs 1.6 inch
Thanks to Carson at Riverside Audi for the tires and TPMS sensors
Major props to my longtime friend Federico Pavoncelli of RD Sport for the awesome wheels