Looking at some 17inch rims..
The offset is 40... Tire size is 205/40/R17...
These should fit my 1990 GXL fine correct?
It's comming off a Ford Probe and i believe they have the same offset as FC's...

 

They should fit, but the tires are too small. The diameter is too small. A much better size would be something like 215/45/17, that'll get the diameter to within half a percent of stock, which is very close. There might be problems running wider tires though as for wider tires you should have lower offsets; however they might work. Do you know what width they are, that will have an effect on the fit, although they're probably 7" right? I know for sure that with a 93 MX-6 (cousin to the Probe) the bolt pattern is exacly the same, and the centerbore is bigger, so they should bolt up just fine.