Member, P500 Sport, DSC off (>900)
Join Date: Oct 2009
Location: New Zealand
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From what I've heard and read on other forums, the E38 wheel (ET23) is marginal for the E24 and only works without a spacer if you use a narrower tyre (205/215).
So if you want wider tyres you'll need a 5mm - 10mm spacer, which is fine provided you use longer wheel bolts.
Also be aware that the front hub configuration of the E24 is pretty much limited to an 8-inch rim width, while the rears will accept wider rims.
The 16-inch wheels from the E28 M5 will fit, but I personally do not think these are very flattering to the E24 coupe.
If I had an M635CSi, there would be no question: I would go with the 17-inch 'Style 5' upgrade, and my 2nd choice would be Alpina rims, like those on the red car in the preceding picture.
I would definitely not go bigger than 17-inch on an E24.
On a non-M version of the E24, the 16-inch cross spoke from the 7-series is a good choice, IMHO.
1987 E28 M5 (DC98) Delphin-metallic/Midsilver Nappa
1986 E23 745i/M88 (6928) Arctic-metallic/Oyster Nappa