Fitment issues
 

I am running 18x8.75 et.20 XXR 527 rims square on my 2015 Brz. I found a deal at Discount Tire that, with my friend, we're planning on doing. I am currently running 225/45/18 square on my brz, but I want to go a size up and because my front left tire has an unrepairable hole. He wants to go 265/40/18 square on his BMW M3, so he will give me his fronts 245/40/18 and I just have to buy a new pair for my rears. I am skeptical though because of fitment issues. I was wondering if it will rub and if so what can I do to prevent that.

Get coilovers, it deletes the huge spring perch that you'd rub on. I had XXR530s in an 18x8.75 with 235s and they rubbed on full lock.

245/40s are a bit tall for 18" wheels. Even 225/45-18 is a bit tall. You want to shoot for 245/35-18 if you want to maintain factory overall diameter

Otherwise, like already mentioned, either get spacers or switch to an aftermarket system where the damper/spring perch is narrower