Regarding this faq about rims:

Why won't my new wheels fit over the rear hubs on my 4th Gen?

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WHY WON'T MY NEW WHEELS FIT OVER THE REAR HUBS ON MY 4TH GEN?

The 1993 to "late" 1996 4th Gen F-Bodies have a larger hub on the rear axle than on the front spindles. The rear axle hubs are about 0.03" larger in diameter than the front hubs. This problem was corrected in late 1996 model year or possibly in early 1997.

As a result, if you buy C4 or C5 wheels, or if you buy 1997 or newer F-Body wheels, they may not fit on the rear hubs of your 1993-1996 F-Body.

The solution is very simple. First, DO NOT FORCE THEM ON WITH THE LUGS! Very dangerous, and you may not be able to get them off. You need to lightly sand or grind (e.g. - Dremel) the inside diameter of the hub hole in the back of the wheel. Don't go nuts.... just remove enough metal so the wheel cleanly fits over the rear hubs, with no interference.

Think about doing this to all 4 wheels, so you can "rotate" the tires in the future.

This will not harm the wheels.... you are taking off the thickness of a sheet of paper. Just be sure you do a neat, smooth job. This will also NOT affect the centering of the wheels on the hubs. The 4th Gen wheels are "lug-centric" - they are centered by the conical lugs, not by the hubs.
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The multi-fit rims I about for my 1994 Camaro fit the front hubs with no problems, but of course the rears do not fit. Do I grind the centre hole of the rims to make them fit or do I grind the entire hub area of the rim?

 

Not sure I understand the question. You have two choices:

or, alternatively, you could grind down the diameter of the raised section of the axle hub (not the rim).

 

I have it figured out now, thanks!