In addition to the pics above, more backspace puts more of the tire under the car. Less backspace pushes the tire out more. The measurement really works in conjunction with tire size. With smaller tire sizes, backspace isn't as critical cause you won't be close to rubbing anywhere. With wider tires, your backspace better be right, or you will be rubbing inside the wheelwell, or on the fender lip, if the tire is pushed too far out. Does that make sense?
As for the 15x8's, 4.5" of backspace is about ideal for the back, but your Ralleys are probably 4 1/4" or 4 3/8", which is fine for a 235/60 tire.. maybe even a 255/60. But on front 15x8's (from what I've gathered) really don't work well. Most folks recommend a 15x7, as you will likely have rubbing issues with the 15x8's on front. Some have made it work, but others have complained of rubbing issues. With a narrow tire, the 15x8's may work... like a 205/60 or 215/60. (I don't know how narrow you can go on an 8" rim)
Do a search on "ralley" or "rally" and you will find all sorts of info.
Here's another measurement for you that if you are mixing and matching hubs and such to come up w/ a front disc brake conversion that is called "Joe" after I wanna say, Joe that came up w/ it over on Team Chevelle's site several years ago!!
It is the hub/wheel mounting surface dimension measured from the end of a front hub!
A drum hub and both the "one-piece cast rotor/two piece disc" hub's Joes will be different by like 9/16" per side w/ the drum hub moving the wheels in more!!
David Pozzi's great site has a pic of this being shown between two hubs if you want to do a search..
hey norcoastal dont buy tires yet.i got rally rims 15x8. friend of mine got them on his 68.it is 275-60r15 on back. fronts are a 70 series and same height of rear tire.in 2 weeks im going to get bf goodrich tires.ill let you know what size is front soo no tire rubbing.
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