a mechanical issue? Just wondering for the day I finally pull the trigger on 18's...and a VAG-COM setup.

For instance, if you have 215/55-15 tires right now, and you get 225/40-18, the diameter of the tire will still be almost exactly the same, hence no change in speedo readings.<ul><li><a href="http://www.miata.net/garage/tirecalc.html">Very handy tire size calculator</a></li></ul>