Is it ok to run square 225/40R18 on the 2018 stock fsport wheels? I want to use my stock wheels for the winter and run a square setup to rotate. I believe its 18x8 at the front and 18x8.5 at the back. The stock tires are 225/40r18 front and 255/40r18 back.

Thanks in advance!

 

No. 225 down from 255s is a BIG jump. You'll be stretching the tire a lot and that's not good on the contact patch in winter conditions when you want the most traction. You might be better off finding a similar sized used wheels (either 2 or 4) and putting the winter tires on those to swap.

 

Rears are actually 255/35

 

It's perfectly fine and used by many as a square winter setup on the OEM rims. Just look at tirerack they even list the 225/40 setup as an alternative to running stock size winter tires, You will just have to be a little more careful of the lip on the rear as it will be more exposed.

 

Would it not be a bit safer to do 235/40R18 all around? It would be much less of a stretch in the back (and it would only be .3 inches over stock diameter - negligible difference).

 

I don't think 235's are going to be any more "safe". 225's will be totally fine, work arguably better in the snow, cost less, and have more availability of tire choices (being a more common size choice). That being said, I went with OEM 225/f 255/r winter tires on mine. I don't care too much about rotating them, and usually one set of tires will get me through my (often short) lifetime with any given vehicle.

 

Yep, OEM size would be the best bet naturally...

 

This looks to be the start of a bad idea all around.
Simply putting the same size tires on the front and rear doesn't mean you should be rotating the staggered rims.
And because there is enough difference in the front and rear rim sizes, there is no one size tire that is a best fit for both.
If you really want it to handle, grip, and drive better in the winter, put a set of 17 inch rims and tires on, and call it a day.
The staggered rims will be safety stored away, and you won't be worrying about anything.

 

Having owned a 2008 IS 250, I wasn't a huge fan of the staggered wheel setup once I started living with it (tire wear and alignment issues). Interest has piqued again in the IS and was wondering if I could run a 'square' setup all the time on the IS models that come with a staggered setup. The only IS that's available from the factory with a square setup is the base IS 300 as even the IS 300 F Sport models come with the staggered setup.

I'm wondering if I can just run the square setup on the F Sport versions or is the base IS 300 mechanically different from all the F Sport versions? I don't mind losing some performance (if any) from the staggered setup.

 

Yes you can run a square setup on the IS, even if it came factory with a staggered setup.
Unless you're tracking the car or driving really aggressively on the city streets, you won't, or shouldn't, notice a handling change...depending on what square setup you put on of course.