[QUOTE=Bill Verburg;6734347]...255/40 x17 is a much, much nicer compromise[QUOTE]

That's what RUF and myself choosed:D

Exactly... How can that be right, when all of the suggestions for the same setup (SC hub w/turbo wheelarches) are running MUCH less offset?
If I understand the inputs here, I should be running a total offset (w/spacer) of not more than -8 to max -10.
Maybe I'm assuming that I have the old RUF speedline ET, but actually have the new ET?

Hope I'm not "stealing" the thread here...

I have 10 ET-15.3 on my 930 without spacers, that's what RUF did on 930 too. Mine are according Strosek (same offset like 928)

Don't know if this is the same for SC's with turbo flares as there are different banana arms

They provide the info you need, you can also put the info in the app I sent to get a rough idea of the height of a tire, it's only a rough approximation because the actual as delivered specs vary depending on the manufacturer and the actual wheel the tire is mounted on.

if you wanted to run the Avon CR6ZZ 245/60 x15 you would want an 8.5" wheel ET7, that places the outer face of the wheel ~.35mm more inboard than Jeremy's and the tire sidewall 5.35mm more inboard than Jeremy's

to run the 295/50 you would want a 10.5" wheel ET-14, that places the outer face of the wheel .35mm more inboard than Jeremy's and the tire sidewall the same 5.35mm more inboard

also note that assuming Jeremy is running a 315/35 x17 the 295/50x15 is ~.93" taller w/ commensurate influence on gearing

A true 930 and and Sc w/ 930 sheetmetal have very different wheel needs.

In front it varies by year because of the different hubs used, '75-80 930s used 911 hubs and a separate 21mm spacer which can be discarded, but from '80.5 on the 930 hubs themselves had 21mm more o/s built into them.

In back the 930 trailing arms also have more o/s built into them I'd need to look it up but my increasingly failing memory says that the 930 arms have ~2.25" more o/s than a 911 arm

when the hubs or suspension has more o/s wheels of the same width need less o/s to fit the same

Oe Ruff 10s used on 930 chassis were ET5

what ET you need depends on the suspension, sheetmetal, wheel & tire width

What others used on a similar setup is a great starting point but there are so many potential pitfalls because of individual instillation details that it's only a starting point.

My advice once the fenders & suspension is installed is to fit a well defined known wheel/tire and then measure how much further you want it to sit outboard.

My other concern which may be a non issue is the effect of increasing offset on the scrub radius, particularly in front. We can assume that what the factory does is ok and try not to deviate too much from there.