[crocair]

I just picked up a 2018 i3S Rex yesterday. It has brand new front tires but the rears are worn out pass the tread wear indicator. There are some 215/45/20 tires on sale for $100 instead of the standard 195/50/20 bridgestones at ($320) . I searched, and those will fit on a 2018 i3s rear wheels and normally one would upsize the fronts to 195/50/20 that came from the rear.

My question is if the front stay at the 175 stock size. What would be putting the 215/45/20 in the back feel like, or throw any codes? Would it be unstable? I picture a old hotwheels car with wide tires in the back and skinny tires in the front.

[bagd0003]

You can use the tire size comparison tool. https://www.discounttire.com/learn/tire-size-calculator.
In your case, the overall tire diameter difference is negligible. As long as 215 wide tire is ok to put on the rim width that you have, it should be fine and shouldn't throw any codes. The car will probably be more stable, but might affect the range slightly.