same tire load index, runflat vs nonrunflat

Is there an inherent difference between a 91v rft vs 91v non rft?

I know rft's are stronger, but 91v means it still shouldn't be loaded above 1356 pounds right?

Why is the 2009 335i convertible riding 91 load index oem tires? and if my car is lighter, can I run 91 load too?

I noticed on the 2009 bmw 3 series convertible, which weighs 4000 pounds curb weight, runs on 91v runflat tires.

My car is lighter than the 3 cabrio, at 3800 pounds curb weight, and the factory put on 93y 245/40r18 tires.
4600 lbs gross weight, 53/47 weight distro.

I want to downsize to 225/45r18, but most only come in 91y standard load.

door jamb does not state that I need 93 load index, and my owner's manual says to pick a tire that is more than half the gross axle weight.
I'm assuming that is 4600 x 0.53 / 2.
So the 91y tire appears to be sufficient.

I never have any passengers, so my car is usually under 4000 pounds gross.

Is your question "can I use 225/45R18"?

If so, this size is actually taller (not a "downsize") than the original sizing and may rub.

my question is more "can i run 91 rated non rft tires ?"
Summary- I'm trying to justify running 91 load by comparing my car weight with the heavier 335 convertible weight. However, the difference is that the 335 is rft, and I am non-rft.

my car- 3750lbs curb, 4600 gross, 53/47 distribution.
335i convert- 4000 curb, 4800 gross, 48/52 distribution.

Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Difference
my factory sizes:
245/40-18         3.9in  12.9in    25.7in         80.8in        784        0.0%
225/50-17         4.4in  12.9in    25.9in         81.2in        780        0.0%

my pick:
225/45-18         4.0in  13.0in    26.0in         81.6in        777        0.4%

yeah, i know it's a tad taller, and it doesn't rub. I wasn't worried about that.
I used 'downsize' to mean narrower, and 2 load ratings lighter. I'm downsizing because I can save $50 per tire.
The standard tire that comes with the car is 225/50r17 93v rft's or 94w nonrft.
Factory 18" option comes with 93 v/y nonrft.

I'm trying get validation that i can run 91 load by comparing the curb & gross weight of my car to the weight of the heavy 335i convertible.
However, the 335i only comes with runflat tires-
91w rft 17".
Factory 18" option comes with lower 88w and 90w load rft's.

I am wondering why such a heavy car can run such low load ratings, and is it because they are runflats?
lack of spare tire aside, can the 335i run the same load nonrft tires safely ?
...because if the heavy 335i can run 88/90w nonrft, I think I can downgrade my 200lbs lighter car from 93y to 91y safely. Is this a safe assumption?

Load capacity is fine and not a concern in this case. Unless you have a different vehicle that you haven't told us about...

Your information on what you think came OE on an 09 335i car under your chart titled "my factory sizes" is incorrect. Stock sizing would be either 225/45R17 or 225/40R18 and 255/35R18.