I HAVE 18" X 8.5" WITH 35 MM OFFSET ON A 2003 AUDI A6 3.0 QUATTRO. MY TIRE SIZE IS 235/40/18. I GET A VIBRATION BETWEEN 55 AND 75 MPH. WHAT COULD IT BE BESIDES THE CAR OR A BALANCING? IM THINING OF CHANGING THE RIMS TO 18" X 8" WITH THE SAME TIRE. SHOULD THIS HELP?

 

did you get the wheels balanced??? 55/70 is the norm for wheel wobble, also check if they are rotation tyres and on the right way, could also be you need spigot rings to balance the the wheels through central velocity from the hub.

http://images.google.co.uk/images?sv...+rings&spell=1

 

I actually had the same problem and it turned out to be a bent rim.
It could also be a warped rotor, how are your brakes?

 

Here's a tip... (first, please stop yelling)

Follow the link in my signature to my registry page. Then, click on Stoney's Links. Then, search for "Hunter roadforce balancing." This will take you to the Hunter website where you can learn about this high-end balancing system (including a dealer locator so you can find a roadforce equipped garage near you). This system has solved many people's vibration woes and is an excellent diagnostic tool for troubleshooting these problems (inc. bent rims).