Been looking at wheels recently, found all this very confusing. But then I found this, I wont take any credit for it, only posting so it can help someone else. BMW 1 Series E81 04> 5x120 38-45 205/55/16 225/45/17 225/40/18 235/35/19 3 Series E30 Excl M3 82>90 4x100 20-30 205/55/15 205/45/16 205/40/17 3 Series E36 93>98 5x120 30-40 205/60/15 205/55/16 225/45/17 225/40/18 235/35/19 Optional Rear Upstep 240/40/17 255/35/18* 265/30/19* 3 Series E46 98>05 5x120 30-45 195/65/15 205/55/16 225/45/17 225/40/18 235/35/19 225/30/20 Optional Rear Upstep 255/35/18* 265/30/19* 3 Series E46 M3 99> 5x120 20-25 225/45/18 225/40/19 255/30/20 Rear Upstep 255/40/18 255/35/19 285/25/20* 3 Series E90 05> 5x120 30-40 205/55/16 225/45/17 225/40/18 235/35/19 225/30/20 Optional Rear Upstep 255/35/18 265/30/19 5 Series E39 96>02 5x120 15-25 205/65/15 225/55/16 235/45/17 235/40/18 235/40/19 Optional Rear Upstep 265/35/18 265/35/19* 5 Series E60 03> 5x120 15-20 225/55/16 245/45/17 245/40/18 245/35/19 255/30/20 Optional Rear Upstep 275/35/18 285/30/19 285/25/20* 6 Series 04> 5x120 15-20 245/50/17 245/45/18 245/40/19 245/35/20 255/30/22 Optional Rear Upstep 275/40/18 275/35/19 275/30/20 295/25/22 7 Series 02> 5x120 15-25 245/55/17 245/50/18 245/45/19 245/40/20 245/35/21 255/30/22 Optional Rear Upstep 275/35/20 295/30/21 295/25/22 Z3 96> 5x120 30-40 205/60/15 205/55/16 225/45/17 225/40/18 235/35/19 Optional Rear Upstep 245/40/17 255/35/18 265/30/19 Z4 03> 5x120 30-40 225/50/16 225/45/17 225/40/18 235/35/19 Optional Rear Upstep 245/40/17 255/35/18 265/30/19 X3 04> 5x120 40-45 235/55/17 235/50/18 255/40/19 245/35/20 X5 00> 5x120 40-45 235/65/17 255/55/18 255/50/19 265/45/20 285/35/22 305/35/23 295/30/24 the offset is all about how far the face of the wheel is from the hub. So as you said the smaller the offset the closer to the arch. The larger the offset, the closer the face of the wheel is to the hub, therefor creating more space between the face of the wheel and the arch. You also need to take into account the width of the actual rim. E.g a 9.5" rim with and offset of 45 will sit closer to the arch than a 9" rim with the same offset.