i was wondering i could put 225/40/18 on my audi with out messing up my speed gauge?
yes.....all german cars (maybe other euro companies too) have a small amount of speedo error built into them. There's a law in Germany that says a cars speedo cannot ever read slower than the car is actually going, thats why german cars, BMW's and Porsche's most notably are know for indicating speeds significantly higher than the car is actually going (at higher speeds (85+mph), where the rolling diameter of the wheel really comes into play), this is so if the owner puts bigger wheels on, yes it will affect the speedo slightly, it will never read slower than you're actually going.
Just for references sake, I got this info from an investigation C&D did into this topic a year or so ago.
A forum community dedicated to all Volkswagen owners and enthusiasts. Come join the discussion about performance, builds, modifications, classifieds, troubleshooting, maintenance, new releases, and more!