2018 i3S wider 20-inch wheels

i3s 20" wheels style 431 are listed as:
front: 5.5x20 ET:33    paired with 175/55 R20 tires
rear: 6 x20 ET:39       paired with 195/50 R20 tires

In contrast, i3 20" wheels are listed as:
front:   5 x20 ET:43    paired with 155/60 R20 tires
rear: 5.5x20 ET:53    paired with 175/55 R20 tires

Data source: www.realoem.com/bmw/enUS/showparts?id=7Z63-USA-01-2018-I01N-BMW_i-i3s_94Ah&diagId=36_2452

To answer your question, i3s has +0.5" wider wheels and +20mm wider tires, front and back.
Given how narrow non-S wheels are, it would be a challenge to squeeze +20mm tires onto them.

I am not prepared to say it would be impossible (tires would be too wide for the wheel, but physically, may fit), but BMW clearly thought it needed another +0.5" of wheel to fit the 4.5-5.5".

Since the only source of wider 20" tires are BMW dealers right now (not available from TireRack yet, and no other 195/50-20 tires is on the market), you would have to (a) over-pay through the nose at the dealer for the tires; (b) convince them to go along with your experiment.

Good luck on both counts!


a
P.S.:  19" are 5x19 all around.In contrast for both i3 and i3s.