Facts and opinions: I have a 70GS455 bone stock. Looking to get 15" rallys to accommodate disc brake upgrade. What are considerations in deciding between 15x7 versus 15x8? Im not racing the car. Is it simply: if you got the cash go for the 8". Would I be locking my self in to a small and expensive tire selection? Thanks in advance. Also, is there any doc, webpage or thread out there that catalogs the difference between the rally wheel codes?
Wider tires require a wider rim, so it depends on what tire sizes you intend to run. The optional tires on the GS 455 was a G60-15. The modern day equivalent of that is the P245/60R-15. A 7" wide rim is fine for that size. Having said that, there is no down side to running the 8" rim on the 245 tires. More support for the tire side walls, and you can run up to a 275 or 295 tire in the back if you like. 8" rims are aftermarket only, Buick only made a 15 X 7. I have 7" in the front with 255's, and 275's in the back on 8" rims.
From a handling standpoint, most experts recommend a wheel that is the same width, to 1" wider that the rated TREAD width of the selected tire. This assures full tread contact. Most competitive road race cars are set up like this, at least those that race on paved surfaces. In the past I have run a 245/60r15 Radial TA on 15x8.5" Centerline wheels on the front of a 70 GS, and there was noticeably less under-steer, but it was not totally eliminated.
Every tire size has an "approved" or "designed" rim width. Tire Specs should give you that information. There is also usually an "alternate" rim width listed, however, the designed rim width is always best.
