TIRE SIZE- How it works?

Hello everyone! I have a question for tire sizes, if anyone can answer would be very helpful for me and everyone else that is interesting in tuning!
How tire size works in the game? The same like in real life or just rim size is important?
For example tire size …

  1. 195/55/15 is with diameter in inches (mm) = 23.44 (595.5) - 3
  2. 195/50/16 is with diameter in inches (mm) = 23.68 (601.4) - 5
  3. 195/40/17 is with diameter in inches (mm) = 23.14 (587.8) - 1
  4. 195/35/18 is with diameter in inches (mm) = 23.37 (593.7) - 2
  5. 195/30/19 is with diameter in inches (mm) = 23.61 (599.6) - 4

From that example we can see that 16" is a largest and 17" is the smallest one, BUT is that really matter in the game or 15" would be smallest and 19" would be largest one.
How it is work?
Why it is important to know? Imagine that all of this would be the options for front tires and in rear we have 255/20/19 with diameter in inches (mm) = 23.02 (584.6), so if I choose to install in front 16" which is largest in diameter in that case but smaller rim size, how would that be affecting the car?
Oversteer/ Understeer - smoler in front larger in rear but rim size or diameter of the tire?
Please if anyone can answer that would be very much appreciated!

Bigger rims can hurt low end acceleration for grip builds. If the build has more power you can get away with the weight issues.
Bigger rims in the front can help with turn in.
Bigger rims in the rear can help some with rear grip but too big on some cars will create instability. Therefore you will see some cars with 18-20 inch rims in front and 15 inch rims in rear.
Bigger rims sometimes drop PI which may be what’s necessary to get the upgrade you desperately want.
Bigger rims hold speed better and as such are recommended for ovals, momentum tracks like the alps, and sometimes top end tracks.
For super light cars bigger rims can help with overall handling and stability. To me it feels like the extra weight keeps the car stuck on the road better and reduces the sliding/instability issues that pop up for some cars. Too big and it’ll kill acceleration.

Usually though, rim size for me is mostly to take advantage of the lower PI. I adjust accordingly if needed.

Is that meen that overall tire diameter doesn’t matter, but only rim size is important.
And yes you are right, lately I see alot of setups with large rim in front and small in rear. You will never see that in real life.

I build my drag cars with bigger diameter front rims than rear. Mostly because it looks draggier, so to speak. I do however use a tire diameter calculator, Wallaceracing.com, to find overall tire diameter and then use that to calculate rear gear ratio. Again, there’s a calculator for that as well.

I have a sneaking hunch that alot of these real world numbers and figures don’t truly carry of to the Forza world. But I have had some success using some of these calculators when tuning gears. I’ve also downloaded a few OEM transmission ratios and I try alot of those out too.

Swerve is a much better tuner than I am so take his advice, just thought I’d throw my 2 cents out there.