We have a 1990 IROC Z28 with a 2000 TA rear-end.

The Sport Max people are telling me that an 18x8.5 with a 35mm offset will work on the front of this car.
They also say I should use the same on the rear, or possibly an 18x9.5, also with a 35mm offset.

Would a spacer be required? A 35mm offset would move the front wheels IN by about an inch, wouldn't it?

 

Which way is the off set + or -?

Ask then what the back spacing is and how they measure it?
Different people measure it different ways. This is a fact. Also alot of the wheel offsets posted are BS.

How wide will it be with the tire?

Measure it that way too.



After market brakes like Baer also change the offset requirements as the move the mounting hub outside.

On the front the constraint will be the strut on the inside and the fender on the out with the 18 inch wheels.

Sounds basic, but with 16 and 17 the tie rod can be the constaint.

DO the measurements yourself.

That is the only way that you will know.

 

Here, this info will help explain what SDIF is saying.
Just because one particular "offset" of rim works on one persons 3rd gen doesn't necessarily mean it will work for you because thay may be running different brake hub setups.

["I have some confirmed offset figures for scrub radius between the different brake setups. All figures are measured with a factory 16" "Fronts" Iroc rim with the standard offset of 0mm ("rears" are -16mm.)
Measured from the spindle mount hole for the 1LE bracket going outward.
----------------------------

Standard Non-1LE front -small bearings(10.5" rotor)
3.630"

1LE brake setup- large bearings (12" rotor)
*THIS IS THE FACTORY STANDARD FOR 245/50-16 tires on an 8"wide rim*
3.948" [+.318 offset, or just over 5/16"]

My setup-large bearings (12.9" rotor) with a 3/16" wheel spacer for rim to caliper clearence
3.967" [+ .337" offset, or just over 5/16"}

13" C4 HD's on 1LE hub-large bearings(13" rotor)
4.248" [+.618" offset, or just shy of 5/8"]

13" C4 HD's on std hub-small bearings(13"rotor)
3.930" [+.300" offset, or just shy of 5/16"]

C5 setup on stand or 1LE hubs (12.75" rotor)
No clue?If anyone has a "+ or - "offset from any of the combo's above- please contribute."]

 

We have the 5.7L with 16x8 rims, so I suppose that means I have the same hubs as the 1LE setup? The front brakes are stock, and the rears are the stock brakes from the 2000 TA rear-end. The brakes clear the stock 16x8 rims ok, so I'm not worried about clearances on rims 1" or 2" larger.

I will ask about positive vs negative just to be sure, but I had assumed positive offset -- I was under the impression that negative offset rims are rare. Of course, positive moves the outer rim farther inboard. If what I've read is correct, the stock 8" front rims have 0mm offset, which I think means a +35mm offset on an 8.5" rim moves them inward almost half an inch compared to stock.

That seems like quite a lot, which is why I asked.

Figuring out the rears has really given me fits. The stock rear 16x8 have (apparently) -16mm offset. On this rear-end that makes them stick out quite a bit. Someone told me they're 1" farther out than they are on the stock rear-end. That suggests to me that an 8.5" rim with +35mm offset would actually look just about right on the back.

Or have I screwed this all up?

For what it's worth, the rims are Sport Max 962's in gun metal:

 

I just looked at the Sport Max guy's e-mails again -- It isn't that way on the tire shop's website, but in e-mail he always refers to offset as +35mm, so yes, they're positive.

I sat down and did the math (with some help from Visio for visualization) and determined the fronts will work with the 2" billet Skulte spacers. This is really just a daily driver, so I'm not as worried about running spacers as I would be on a track car.

Unfortunately, the rears on 8.5" rims will sit as far-out as the stock rims do on that TA rear-end, but it won't kill me. The spacers will push the fronts out a little, so I'm hoping with the slightly wider tires all 'round, it'll end up looking more or less right (or at least proportional).