Largest diameter tire to fit 550xi sport package

If you have 245/40-19 now, going to 245/45-18 are virtually the same size. Same width and a total diameter of 26.72" (former) and 26.68" (latter with 18" rims. Only 0.04", less than 1mm . If you want to stick with 19" rims, you will need a aspect ratio of the same, 40 to maintain size. Going to 245/45-19 is very nearly a full inch bigger at 27.68" (0.96") and that may cause problems with other things. I am going to direct you to another thread the we went through this on a BMW, I will put it in a separate window to make it easy to hyperlink. Changind only a little caused a lot of problems. 245/45-18 is stock for the BMW 550i xDrive This is a different BMW ..just an example of what all we went through. Best to stay as close to stock as possible, and that is 245/45-18

http://www.cargurus.com/Cars/Discussion-t24656_ds665437#msg460711