[8D] I'm looking to replace the wheels on my 03' A4 3.0. It's got the 17' s on it right now. But I'm looking to go bigger. Should I just go with a lower profile tire on 18's so that I don't change the overall diameter or does it even matter? Does it make all that much difference with speedometer inaccuracy, or mess with the computer if I go with 18's and keep the same profile tire? (basically expanding the overall diameter by 2' or so.

 

This tire-size calculator should demonstrate, for you, the answer to your question
http://www.miata.net/garage/tirecalc.html

But, basically, if you go UP in wheel diameter you MUST go DOWN in profile aspect