Will running a different size spare hurt the diff?
 

My X5 has no spare at the moment and I'm taking the family up the road about an hour tomorrow. I have some spare 17" e46 wheels - can I throw one in the car in case of emergency, or will the different diameter cause trouble with the diff/other? We're talking about 3-4" of total difference. Can always take another car, just could use the space of the X.

Thanks guys, Merry Christmas.

Not at all good for the differential. Can also cause DSC problems as the system tries to compensate for what it perceives to be a spinning wheel.

You should try and keep the overall diameter of the circumference of the tire the same. On a 17" wheel BMW used 235/65R17 size on the E53; on 18" 255/55R18; and 19" 255/50R19 and 285/45R19. All have very similar overall diameters on the outside perimeter.

Strange that you can get the sport options that comes with 18" wheels, but the spare is just 17". What were BMW engineers thinking?

That if the OD is the same (tire), the rim diameter is meaningless provided it fits over the brakes.

And if you're carrying it 'for if', it's better than not having a spare. Limp it to the next town and you'll be fine.

17 inch spare 'rolls' at the same rolling circumference as my 19" 132s...
No idea what the OP's E46 17 wheel rolls at.

Agree with audiophool: toss it in, better than walking. Or, cell phone for flatbed, or a simple plug kit and compressor. But, an hour ride? Would not give it a worry.

Fwiw, we did a 13,000 mile Road Trip in our VetteVert with non RFTs, a plug kit/compressor, and the tires never blinked, other than getting worn down, lol!

I wouldn't wring my hands; last flat/nearly every flat I have had, was noticed in the driveway when car was parked.

Thanks for the feedback fellas. Lots of good points - it's a short drive, people on both ends to help, etc. Only reason I'm giving it a second thought is that we've got a fairly new baby and none of us would appreciate being stranded tomorrow. I've actually never had a flat on the road in 15 years of driving, just figured I'd be prepared if possible.

Not sure about the wheel size talk, spares are often different wheel sizes at least from what I've seen. In fact I wouldn't give it a second though also if it were a 2wd vehicle. I was just wondering if running a different diameter would blow up a diff or transfer case in short order, and sounds like the concensus is that I'd be alright if I had to limp along. Cheers.

Listen to JCL.

I did originally say diff, but I was really more concerned about the x-fer case. It's not exactly uncommon for one wheel to spin faster than another, hence the reason for lsd's. I mean the question is really hypothetical in nature anyway - as it would be emergency use only - but what do you think it's gonna do to the diff? Anytime someone is off-road or in ice or snow, or just hot-dogging it, one wheel is likely spinning faster.

If one wheel is spinning faster then DSC applies a brake to slow that wheel. That would suck. You would at least see an awd fault light, depending on whether you have x drive or not. You don't list your year and model, so not sure there.

Does the smaller wheel with a different offset clear your brake caliper?

Do the larger 14 mm X5 wheel bolts fit the E46 wheels designed for 12 mm bolts?

Just seems like a really bad plan. If you are concerned about a flat, take the other vehicle, or rely on a cell phone. My $0.02.