I just received a set second hand OEM RS6 alloys from ebay. I have an A3 8P 2.0 TDI Sport, which has the standard 17" Star alloys, which are 225/45/17 on both front and back. The new wheels are 225/40/18 on the front and 235/40/18 on the back. I'm a bit concerned that the front and back wheels are different sizes on the new wheels but the same on the old wheels. Can anyone shed any light on this? Have I gone and bought incompatible wheels?