Fitting 18's won't affect the speedo as the rolling radius remains the same as the tyre height decreases with the alloy size increases, as long as you fit the proper ofset then you won't have any rubbage , of course OEM alloys will fit fine as they are proper ofset, the same goes for 19" too I guess, tyre size for 19" should be 235-35-19 iirc, for 18" it should be 225-40-18