I don't think it is wind-up, as with a fixed part time 4wd setup, but I do think that a constant speed difference between front and rear contributes to transfer case wear. The same can be seen with a differential in an axle that is designed to allow for differences left and right, but which can wear out due to constant speed differences. Spinning one wheel can do that quite quickly.

BMW techs on the board have commented that they have seen damage to transfer cases from tire size differentials. I trust those techs. I do know the front output shaft is somewhat weak anyway, due to the limited spline engagement, but will defer to the expertise of a dealer technician.

I have personally only seen DSC errors due to the difference in rotational speeds.

I always thought you installed the new tires on the front as well if you only buy two, but I don't think that is correct.

Tire Tech Information - Where to Install New Pairs of Tires?

Here is a Michelin link. Once open, scroll down to the bottom of the page.

Replacement Tips: Tire-saving Tips: Tire Care & Buying Guide: Michelin Tires

Both those links argue that understeer is safer for most drivers than is oversteer. This is the same thinking automakers use, for liability reasons. They give up steering control on the basis that the average driver will not know would to do in the event the vehicle oversteers. It is the dumbing down of car control. I understand their logic, but I prefer to maintain control and I can do that better if I still have steering. Maintaining steering means putting the best tires on the axle that steers.

Yeah, but what good is steering the front of the car if the back comes around?

I agree that their logic is understeer is better than oversteer for the bulk of drivers on the road. It probably is. More drivers will be able to correct understeer than oversteer. Pretty much all of the tire manufacturers recommend the new tires to be installed on the rear.

Here is a quick video. There are tons more online.

It allows you to steer into the skid, while feathering the throttle, so that you can flat track it around the corner.

On the other hand, if you understeer into the corner, you have lots of spare time so that you can just hang on and enjoy the view as you sail into the trees.

Edit: I agree that a skilled driver will only be able to do so much, ie recover a certain amount. Past that, he or she is going to end up in a similar situation to the understeering vehicle. I also wasn't suggesting that the old tires would be bald, but rather just partly worn.

All of this changes somewhat with DSC. If you understeer, there isn't much it can do. If you oversteer, it can apply a single brake very quickly to help keep the car under control, within the limits of physics. It still alters the equation, IMO.

Everyone should go to (pavement) skid school. Slightly off topic, but I recall driving an overpowered Ford Sierra rally car on a dirt track, with an instructor. Halfway through the corner he could flip a switch and lock up the rears, and then watch as we got ourselves through it. Everyone should have so much fun.

so basically, if you can't drive, put them on the back. if you can, put them on the front.

I agree with jcl, driving schools are a must. i have taken many myself, they are well worth the cost.

I hear you, but I feel the tire industry recommends new on the back because most people are not as good of a driver as they think they are and that for MOST understeer is better. I wouldn't want to see the average driver try to feather the throttle to correct a skid. :wow:I am not even sure if the average driver knows that they should steer into a skid.

Now, it would have been great to be in that Sierra though. Fun in a controlled envoronment with an instructor is one thing and driving towards a Toyota Avalon that is sliding sideways around a corner while the driver is on his cell phone is another.

Which is all pretty much irrelevant as most x5`5 have different tyre sizes on the rear to what goes on the front so cant be changed around anyway.

Wear to the planetary gears, yes, quite possibly, but that's a different matter.

Commonality is not causality :) These propshaft splines are known to fail anyway, how anyone can claim it's definitely due to tyre size in some cases, but not in others I don't really know. From an engineering point of view, a small difference in tyre size front-rear will have absolutely no significant effect on the loading on this spline. IMO far more likely is a different connection - that the kind of person who fits huge blingy wheels is probably the same kind of person who drives their car hard, doing full-bore launches more often. That will shorten the drivetrain's lifespan a lot more quickly than having slightly different tyre sizes !

Same here.

No, most X5s are 3.0 gasoline models with 17" wheels.