dpgx5 provides some good information on space saver wheels.

In short, if your car was 3.0 (gas or diesel) or 4.4 and was ordered with the space saver wheel, you would have the 18" version: BMW 36 11 6 750 316

The 4.6 & 4.8 with the larger front brakes (which take a mininum 18" NORMAL wheel size) would have the 19" space saver version: BMW 36 11 6 756 858

If you upgrade to larger brakes which requires a wheel (size) upgrade from 17", remember to check what you have for a spare tire. Usually a take-off 18" or 19" normal wheel is a better value alternative, unless you really want to save a few kg or lb in the trunk.

dpgx5 provides some good information on space saver wheels.

In short, if your car was 3.0 (gas or diesel) or 4.4 and was ordered with the space saver wheel, you would have the 18" version: BMW 36 11 6 750 316

The 4.6 & 4.8 with the larger front brakes (which take a mininum 18" NORMAL wheel size) would have the 19" space saver version: BMW 36 11 6 756 858

If you upgrade to larger brakes which requires a wheel (size) upgrade from 17", remember to check what you have for a spare tire. Usually a take-off 18" or 19" normal wheel is a better value alternative, unless you really want to save a few kg or lb in the trunk.

If you can find a 4.8is spare you're ok.
If not, I think it's not the end of the world. Think about it. How many times in your life you had a flat tire that was bad enough that you couldn't drive to a garage or a place with a compressor? Then, take that low percentage of chance that this will happen to you, then divide it by 2 (50% chance that happens to the front vs rear). That's not a lot.

Of course you need to be able to drive to a garage IF this happens. What can be done is just bring a small 12V compressor, to inflate enough to go and repair the tire, or, install the spare tire on the back wheel, then, the back wheel to the front wheel. That will at least let you drive to the nearest garage.

I have the Brembo BBK and removed my spare as there is no way it is fitting over the front brakes. I decided to buy extra towing coverage instead.

I wonder if a 20" style 87 or equivalent would fit in the spare tire well?

I'm in the same boat as my spare for the 4.6iS is missing. The dealer put a 17" standard in their and I never noticed for a year. A little late to go back. I've been starting to look for an option other than the factory since they are really expensive now.

I have successfully used an aerosol tyre inflater with temporary repair properties. You might consider that as a safety net if you go spare-less...

Tyre Repair :: Repair & Maintenance :: Holts Auto

Nope 20" will not fit.