Should I downsize from 245's to 235's?

[Spence96]

I'm running XXR 530's 18x8.75 on my BRZ with Tein lowering springs. (I am a college student on a budget) I have had a set of Michelin PSS 245/35/18 and I am now currently on Continental ExtremeContact DW 245/35/18. With both sets of tires I feel like my sidewalls are flexing under cornering during spirited backroads driving (60-90 MPH) although the Continentals are much worse about it than the Michelins ever were. My car understeers then suddenly snaps in the direction I am turning the wheel, and it is unpredictable and sketchy. I think my tires may be too wide for the wheel I have which may be causing this issue, which is why I'm considering going down to a 235/40/18. Can anyone shed some light on this issue and educate me? Thanks if you read all the way through this!

[Scrappydoo]

Nobody say "battery".

[churchx]

I'd check first thing tire pressure, easy to do yourself at any petrol station.
Next thing i'd check alignment at some competent shop. For example - slight toe-in should help with stabilizing rear.
Also a bit weird to see 18" chosen & then mentioned budget. Bigger size/lower profile tires will be anything but cheaper.
For x8.5" roughly & approximately Minimum 225 / Ideal 235-245 / Maximum 255.
Going for narrower tires for slight stretch / to dial out some sidewall flex will result in a bit sharper turn-in/handling at cost of some loss of comfort. I don't think it will "fix" your described handling issue .. i would rather check pressures/suspension/alignment for some weirdness.
Tires change .. imho tires itself are not really an issue. If anything, they are much grippier due better type/compound/width and loss of grip may happen at higher speeds. But if it should be less progressive, more snap oversteer due grippier tires? Imho not. I'd still prefer narrower tires for car to be more playful at safer/slower speeds (especially if car is still NA, without forced induction), and that narrower should be lighter and cheaper .. but i wouldn't switch them now in hope to "fix issue", only i'd consider next tires bought narrower for said reasons after current set should be changed out due wear. You mentioned budget, didn't you?
P.S.
Those springs .. by what extent they lower? If more then by 1" then imho you should install diff riser at back to lessen rear axles joint wear and roll center adjustment at front, to compensate overlowering changes/ill-effects to suspension geometry.

[why?]

Unless you somehow got a bad set of tires, which could be a possibility, it is not the tire. Do what churchx said, that will probably be where the issue is.

Originally Posted by Scrappydoo

Nobody say "battery".

oh that's no fun.

[Mr.ac]

Tire pressure, and dad driving skills.
Your braking too late and turning at the same time. Try braking one second earlier.

[OND]

Btw those are pretty good tire choices for a college student on a budget

[wparsons]

245's on an 8.75" wheel is about perfect, and based on what you're describing isn't caused by too wide of a tire for the wheel.

It sounds like you're entering too fast and simply understeering (over driving the front tires), then when the speed drops the car finally turns in properly.

I'd agree with checking pressures and alignment as a first step. Also take a look at the tires to see how far onto the sidewall you're using. Most tires have a ridge or little triangles to show where you should ideally be not going past.