What about the spare?
 

Like a lot of folks here,I want to upgrade my wheels.
Pretty modest going from 17" to 18" oem but I got what seems to be a pretty good deal. But only on 4 wheels. So do I pay more for one more (singles on ebay seem pretty high) or leave the 17" spare on the car?

What does everyone else do?

Thanks,
Steve

I had a 17 inch spare with my oem 19s. The spare hopefully doesn't get used much, and if it does its usually short term so having it not match wouldn't be the worst thing for temp. Although I wouldn't be against a matching spare if the price was right, but I wouldn't be overly concerned about it.

Don't worry about the spare - I've got 20s on my X with a 17inch spare. Never used the spare and if I have to use it it wouldn't be for long so not worth the extra money in my opinion

Same here.. I have 20" with a 17" spare. Your 17" and 18" will have the same rolling diameter (or near enough), so it wont matter and like our other good friends that replied, you would only be using it to get home or to the repair shop.

The 18" spare is going to be a little heavier as well, but that is nothing to worry about.

Save your money, keep the 17" spare.

This is only true if you stick with the OE tire sizes, regardless if 17" through 20". All bets goes off if you start veering away from OE tire sizes.

I agree. I don't know how the all wheel drive would get affected, but your if your rotating speed varies a lot, damage can happen. I'm curios if the X would go to limp mode like the S500 does when a wrong spare goes on the drive wheels.

I have got 5% oversize off road tyres (265/65/17 Pirelli Scorpions STR) and had to use the standard 17" spare the other day and it seemed to work OK for the distance I had to do... no limp home mode, and not even a tyre pressure warning (2006 3.0D Xdrive). Only did 20 odd miles and none of it at high speed though... I know not to push this and use the spare only to get home.

5% difference isn't a lot, but the difference will be taken up by your differential/transfer case. If you hook up a computer to your OBD port, you'll see a code logged for too much variation in tire sizes and power/torque was reduced to compensate for the difference.

I had new tires put on the rear with old tires up front and got this power reduced code. I believed the difference was only less than 2% between new and old tires.

Ha I knew it. I was almost sure the old 7 series used to call out this code as well. Yes, such a variation is not much, and I am sure if you get a flat on the spare, you will get a real message on the OBC. Since the X5 is AWD, putting on a different overall diameter spare can affect the entire drive line. I am also sure BMW would not let this be like some old school Lincoln, and built if safeties to prevent damage to the running gear. I am sure if you don't have an option, a smaller wheel will get you home, but I personally would match the overall diameter.

If your rolling diameter is different to your spare then you need to get your speedo recalibrated. Whats the point in getting larger wheels if you're going to increase the rolling diameter (unless you have snow 365 days a year)?