2003 A4 3.0. I'm looking to replace the 5 spoke 17's on it with 18's. I want the same overall diameter that I have with the factory wheels just a bigger rim. So what size tires do I need? What's the profile? Also, what is the offset that I need to keep it the way it is now? I definitely don't want them to stick out from the sides but I wouldn't mind a little wider tire. Thanks.

do some searching and you will find your answer. adn if you have to use SEARCH to fine tune your search. there are plent of threads about this

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-Early '99 A4 1.8T Quattro Tip
-APR 91 -APR Boost Gauge -LLtek Cross Tower
-Evoms CAI -N75H -Stratmosphere Dv
-RS4 Grill -Techtonics Tuning Exhaust

Hold on baby Jesus, it's gonna get bumpy