I have a gc8 with brembos.. 18x8.5+35 offset.. On these rims I am running 215/35R18. they have a flat face and 1.75" dish and still clear the brembos. Clearing brakes is all about the style of the rim. These are mine:

XXR Wheels: 521 18x8.5 Flat Black Rims

The lower the offset, the further away it will sit from the suspension and brakes = more chance of clearance. but ultimately comes down to the style of the rim. 17x8+32 would be the way to go as it has more chance of clearance and also will sit really good with the guards on a gc8

this is a great caltulator:

Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

tyre size regarding speedo issues for a gc8:

Ideal and best tyres size for gc8 with 17" rims would be 225/40R17 or 235/40R17. these are clostest to factory rolling diameter. I have a set of 06 sti rims, with 235/45R17 and the speedo is out wayy too much due to the profile of the tyre.. I think the 235 will be getting close to the guards especially if they havent been rolled, so a 225/40R17 would be better.

I have done the calculator for you, my rim/tyre is the existing (which clears) your ideal setup is the new:

Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

as you can see your setup (17x8+32) will sit 9.3mm further away from the suspension strut. The outer rim will poke out 3.3mm less than my setup. I think this is ideal.

^ as long as that setup clears the brakes you will be sweet and it will look good. Message the company you are buying off and check if they will clear sti brembos, if it does order them, and put on some 225/40R17 will look sweet

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