HabzZ

Hi All, Ive got a 2021 Audi A5 that has 20inch rims which comes as standard in the black editions and was wondering if i could reduce these to 18 inch instead? Ive seen it offers slightly better fuel economy and a comfier ride with the extra rubber. Itd also be more ideal with crappy uk roads. Currently the tyres are 265/30/R20s. Replacing them is not cheap to about £200-300 for new tyres and ive seen 18inch alloys which with the right size wouldnt effect the drive as much i believe? I want to make sure im not causing issues with the Quattro system, braking, speedometer or anything else big on the car. if i went for something like 18 inch alloys how do i ensure they fit my car? What specs should i be focused on to ensure they fit properly? What tyre rubber size would be ideal so i dont create a weird gap between the car and tyres because of the rim size difference? (New to the alloy world really any advice is much appreciated and if you need any further info let me know!!)