I have 17x9.5 Budnicks on the rear of my '01 Firebird. What is the widest 18" rim, tire, and offset I can use without any body modifications? The car will see some track days.

 

Front stick with 9.5" wide, 45 to 56mm offset. Rear use 11.0" wide, 45 to 50mm offset. Whether you need modification with the 11.0" wide in the back varies from car to car. Read the appropriate "sticky" for more info.

What kind of track? The auto-x guys run 11's in the front too, but they require a spacer and the tire edge is outside the fender. You might be able to get 10's in the front, with less than 50mm offset.

 

Road course mostly. Nothing serious just some track days. The 9.5's I have on the front seem to work well. I just want to add a little more grip in the back. I was hoping I could fit 11's in the back. I have an adjustable panhard bar so I should be able to center the axel. From what I've read it sounds like I'll need the 50mm off set.

 

Don't get hung up on 45mm vs 50mm....... that's less than 1/5th inch.