In case you guys are not aware...Going away from the OEM size tires (different rolling diameter) will mean the loss of using the spare (unless, of course, you also changed the spare tire as well)!

It means a standard 235/65R17 spare is less useable, not lost altogether. It can still get you out of the bush if you can't plug the damaged tyre. Only a 5% difference in rolling diameter from 265/65R17 to 235/65R17. Perhaps Cole can tell us if he can fit a 265/65R17 in the well for the spare tyre.

Anyone have similar experiences with an 18" wheel?

I haven't upsized my spare, it is still the original 235/65/17. I didn't try but I don't think the 265/65/17 will fit in the spare well by quite a margin!

If I get a puncture the smaller spare will still be useable after resetting the tyre pressure monitor. I wouldn't use it for long, or at any sort of speed, but like Fraser said it can get you home. I also carry a large canister of Tyre Weld which is an inflaitor with puncture repairing slime inside it. So for small leaks this can temporarily fix the problem without resorting to changing the tyre.

Of course the bigger more robust ATR tyre is also less likely to puncture in most circumstances than a standard road tyre.

It's not just a matter of resetting an "idiot" light (tire pressure monitor), the difference in rolling diameters will put added stress on your entire drive train (especially differentials and transfer case as well as drive shafts if you don't have xDrive).

You could easily offset that by changing tire pressures, if it is a case of getting back out of the bush.

A short distance wouldn't cause any problems, the wheels are designed to move at different speeds.

You just don't want to be putting 50 miles on it going at freeway speeds.

Very true...if it's just meant for getting out of the bushes, then it might not matter. But most people don't realized the potential damage caused by using a single smaller diameter tire, especially on an AWD. Heck, how many times have you seen people drive with a donut doing 80mph+ when the manual clearly specifically calls for no more than 50mph?

I am concerned about possibly having to use my undersized spare as I could conceivably have to do 100's of miles to get "out of the bush" but I was under the impression that the drive train would not suffer that much. I do have xDrive (2006 3.0D), what difference does that make? I understand about the differential having to do more work as the wheels are turing at different speeds but isn't that what the diff is designed to do?

Don't space saver spares normal have the same rolling diameter as the wheel they are replacing? Isn't the reason they advise limiting your speed because of the crappy nature of the tyre?

That is what the diff is designed to do.

What is considered bad is the buildup of heat in the system. If you have to drive a hundred miles with an offset rolling diameter just keep it to lower speeds and you should be fine.

Imagine the heat in a diff from going 120mph on the autobahn, or from a half hour during a track day - I doubt going 40mph with a 5% rolling diameter difference could challenge that.

As for donuts themselves, the reason they limit speed and distance is because it is not designed to handle higher speeds or the weight of the vehicle for more than a short distance.