Have a 2004 M3, with the 19" optional wheels. I know the front and rear wheels are different widths, but could a front wheel be run on the back. I'm thinking of throwing a spare in the trunk for long drives, and wondering if a front wheel could be used on both front and back? Not permanently, just temporarily.

I'm pretty sure you can. It takes up a lot of room in the trunk. I have a space saver spare that I only carry if on trips and that takes half the trunk. BTW, I don't think you can get my kind of spare anymore since they don’t make the tire.

BTW, what year is your ZX? I use to have a 96. :b_hot

1991 300ZX Twin Turbo. Just sold it though, so the M3 has the run of the garage now.

Does anyone use a spare out of a normal 3 series for the M3? Don't know if it would be a match or now...

If you are gonna carry a spare that isn't the correct size, why not save space and use a donut spare?

Where would I get such a donut, is there a particular part number?

I am sure its a standard BMW part that your dealer could order for you.

Here are the part numbers.

99-00-0-003-134 Tire - T125/80R17 $140.00
36-11-6-750-007 Black Rim $90.00

the offset on the front wheel is ET47 and on the rear is ET26. This will cause it to hit the shock. Best bet would be to get a doughnut spare, but you may have a problem with it also. I would find a friend with a spare from a 3 series and try it first. This way you wont waist your money.
Good luck