Seems like the only forum....?

Why did GM usr different offset for rear wheels?
Is it an only an 1/8" difference?

Couldn't you use four fronts and never notice??

I can't give you exact spec right off had, but it's more than an 1/8 inch. Yes you can run 4 fronts, but you may have trouble with the rear tire rubbing the fender(both my rear tires are real close and I'm using the 'rear' wheels out back)

my experience with switching them front/rear is a really bad vibration above 60 mph. not cool.

What is the offset on the rears?

I have them on now- they look great, handle well
and I've had it over 60mph with no vibration-

they do stick out a little but I've seen a 3rd gen witha 4th gen diff and those stick out more

Yep, that pic looks about right. On my car, they would rub--it sets lower and bumps push the tire up into the wheel well. I can't think of how that could ever cause a vibration. If you have a wheel out of balance you might not feel it when it's on the rear--but on the front a small balance problem can be felt in the steering wheel.

The front offset is about 3/8" different than the rear offset. I unfortunately have 4 rear '87 Iroc rims on my car (Got them free and chromed them for $65 per wheel a few years back). The rears on the front need 1/4" spacers to just barely clear the calipers. I found lug nuts that are about 1/8" longer than stock and have always keep new lugs and studs on the fronts after every five or so tire changes just to make sure the threads are strong. I don't have as much thread showing on the front rims with the 1/4" wheel spacers on, only about 3/8' of threads show before I put the lugs on (I get 4 1/2 turns out of the lugnuts-kinda scary- thats why I keep new ones on it all the time).

ah.....so your situation is opposite from mine....
and from what I gather, worse too.