If you were making your own custom wheel, and you went with 8x18 and 10x18 as your wheel sizes...

Any reason not to use the 928 OE offsets?

 

ET65 is fine for the fronts

ET60 is better for the rears in terms of placing the rubber where you want it in the S4 models assuming a 285 section.

The GT is a bit different as it has a 17mm spacer fitted as stock with different studs and then I suspect the bigger offset is absolutely essential or so I suspect. On the GTS I removed the stock 38mm spacer and fitted a 24mm spacer with open lug nuts to slip over the now too long studs.

Two things to note however when configuring custom wheels-
1. If they are modular wheels one cannot always get the exact offset one wants.
2. With larger offset wheels the spokes run very close to the calipers and one needs to ensure that the orbit swept by the spokes does not interfere with the caliper. I have had my fingers pinched so many times when fitting the front wheels [mine are ET68 but they are also 9.5 inch rims]!

 

FWIW 89 GT's have 8x17 et60 front wheels from the factory, all this talk of ruining the handling of a 928 unless it's et65 or greater is all hogwash.

 

The query was "is there any reason not to use the OE offsets" and as I am aware Mr Speedtoys does not have an 89GT.

The bigger offset on the wider wheels is there to get the wider rubber to fit generally speaking. The ET65 is specified to make the car have a stable response if one wheel hits standing water and consequently tries to create a yaw effect- the 10mm negative scrub radius is there to keep the plot traveling in a straight line- I think of that as a rather important safety feature- for sure it has nothing to do with making the car go rounds bends quicker. Why Porsche fitted models like the 89GT and the CS/SE's with ET60 I have never been able to fathom out but one would like to think they had a reason.

Did the 89GT really have a 17 inch front wheel? I was under the impression they had the CS style 16 inch mag alloy rims

 

The Carrera III’s currently on my GT are 8x18 ET 57 front and 10x18 ET 58 rear with 295’s. Perfect fit with room to spare on both ends.

 

I'm running Front - 18x8 - offset 57 with 235/40-18
Rear - 18x10 - offset 65 with 295/35-18 and 15mm spacer
but my rear fenders are custom because of the convertible conversion so that rear setup may not work for other 928s.

At the front, I would suggest not dropping below a 57 offset, especially is the suspension is lowered as you run the risk of catching the fender lip in a turn with a mid corner bump (ask me how I know). If going to a lower number offset you need to roll the front fender lips for sure.

The only reason to stray from the stock offset numbers is for looks. Seems to me if you're doing custom wheels and want to preserve an offset number between 60-65 that you could go to a wider wheel and tire at the front, like an 8.5" or a 9" with 245 or 255 section width tires on them. This would achieve the full fender and sporty car stance look while still preserving the proper negative scrub radius and give you more front end grip. You'd want to pair this front setup with more meat at the back too so as not to make the rear too tail happy due to the added front grip.

 

100% Agreed. Im running both 8x16 and 8x18 et52mm fronts with no handling issues.

 

Recognizing that offsets less than 60mm in front may 'seem' to be OK, the car --really-- loves the correct offsets to take advantage of the rest of the suspension and steering geometry. 60mm was "OK" and a compromise for some of the wider rims. Fitting 8" front rims? Got the choice of using correct 65-70mm offset? Exercise that choice. Do it like you mean it.

 

Yep, hook that fender lip one time and crease the fender and the 911 wheel offset guys (et 50-53) will change their tune. Some learn from others experiences (some positive and some negative) and others just have to experience things for themselves.

My front ride height is lower than stock, but not slammed and I still caught the tire on a set of Carrera IIIs (LWs)