Will 17X8 Mustang rims with 245/45 tires fit on a 87 GXL? I am planning on switching suspension to GC coilovers.

 

no way on the front. depends what offset is for back.

 

sorry to jack the thread but what about 17x7 rims with a +40mm offset?

 

Those wheels will fit, but the tires won't fit in the front, you'll have to go to a 225-45-17, as that should fit. OC apparently runs 235-40-17 up front without problems, but it would depend on the actual tires, as some are wider than others, even though they all say 235, some are more like 225's, and others are like 245's. In the back thay should fit, but they are a larger diameter, so the spedometer will be slightly off, and acceleration will be worse due to taller gearing. The problem in the front is that the tires stick out and rub on the fenders if the tires are too tall or too wide, so the suspention doesn't really help unless you want it to be at a stock hight with stiff springs to keep the tires out of the wheel wells. The only real way to get wide tires to fit up front is to get wider fenders. Good luck