GordonA3

Hi folks, i have a 2010 A3 Cabriolet with standard Audi 17 inch wheels on it (Blue car attached). I would like to change the wheels out for what i think are Audi Le Mans wheels shown on the white car. My question is that going by the hand book, my wheels / tyres are: 245/45R17 7.5Jx17 ET56 PCD 5 x 112 It looks like the Le Mans wheels as an option when the car was new were: 225/40R18 7.5Jx18 ET54 PCD 5 x 112 I am finding it difficult to get second hand wheels from an A3 with an ET value of 54 but they seem easier to come by if they come off an A4 where the RT value looks to vary from about 32 up to 42. So my question is, what Le Mans wheel sizes can i fit? Thanks Gordon