Looking at the sizes below, taken from http://www.vipalloywheels.co.uk/bmw-wheels/alloy-wheels/bmw-3-series/f30-ff/5470/19-inch-bmw-alloy-wheels-style-407-star-spoke-f30-f31:-
"Genuine (OEM) 19" BMW 3 series F30 F31 Style 407 star-spoke alloy wheels (new)
Size: front 8J x 19 ET36 und rear 8,5J x 19 ET47"
if you currently have 8J fronts and 9J rears then the centre line of each front wheel will be 7mm closer inboard, the centre line of your rears will be also be 7mm closer inboard.
This is a good site for working things out, https://www.wheel-size.com/calc/?wheel1=205-55-19X6.5ET45&wheel2=225-45-17X7.5ET40&fcl=50mm&wcl=30mm&scl=50mm&sr=0mm
but there are others. OE wheel on the left, alternative on the right. Put your own numbers in and play around with tyre sizes too. It's easier than it first appears.
