What is the widest tires that I can put on my 17" stock rims. The OE is 235.

 

What engine? For narrow body A6es (2.8, 3.0, & 2.7T), the max recommended width is 235. People will tell you that you can go wider, but for every person that does, you'll find another telling you they had issues. You decide.

For the 4.2 engine (wide body), I believe that 255 is the max.

The other determining factor you do not mention is the width of the rim.