I was woundering if these tires (225/40/18 tires) on 18" rims would allow me change rims without messing with the odometer calibration. thanx a lot.

I have 18in Volk rims and my tires are 225/40/18. I didnt have mine recalibrated. I just told my wife to drive 60mph and I matched speed with her, and my MPH was perfect. So, you should be okay. Now it all depends on the brand of rims you got, because some of the off-brands are much heavier then the others, but all-in-all, you should be just fine.