changing front tires only

[duckseatfree]

So I ordered some Goodyears for the front set of the x5 4.4 at a discount tire. When I came in they were trying to tell me that I had to get all four this changed because it could damage the the tranny. When I told them that was impossible , they said that it could ruin the transfer case. I do need to change the back set but I still have about 15k to go on those.

I'm thinking since the back tires come bigger than the front, it won't effect anything.
Is there any truth to what they are saying?

[duckseatfree]

To clarify. I have the 4.4i 19in sports package

[SlickGT1]

Tell them to thanks for the tires and go home. There is certain amount of slip built into the transfer case to allow for slight overall diameter differentiation. I mean you don't think BMW wanted you to maintain perfect cold and hot tire pressure all the time. They are trying to sell you shit. That's all.

[dkl]

Originally Posted by duckseatfree

I'm thinking since the back tires come bigger than the front, it won't effect anything.

You have wider tires in the rear, but the overall rolling diameter is the same as the front (the circumference of your 285/45 should be very close to 255/50). It should be OK to mix new fronts with old rears or vice versa as long as they're the same brand and model.

[duckseatfree]

Originally Posted by dkl

You have wider tires in the rear, but the overall rolling diameter is the same as the front (the circumference of your 285/45 should be very close to 255/50). It should be OK to mix new fronts with old rears or vice versa as long as they're the same brand and model.

Even if that wasn't the case, wouldn't the DSC light come on?

[dkl]

Nope...DSC will not come on. I'm currently running old Michelin Latitude on the front (255/50R19) and new Pirelli Asym Zero in the rear (285/45R19) - OE sizes. I don't have any warning lights on my dash, but when I hooked up an OBD reader recently, there was a fault code indicating "reduction torque intervention because of too high tire size tolerance". I still need to take care of this by ordering new front tires matching the rears.

[JCL]

Originally Posted by duckseatfree

Even if that wasn't the case, wouldn't the DSC light come on?

No, they are not lying to you, and yes, if the difference in rolling diameter is too great front to rear the DSC lights will come on.

A rule of thumb is to be within a few revolutions per mile of difference. More than that stresses the transfer case. If the rears have more than 50% tread depth remaining you should be fine.

[ard]

Originally Posted by JCL

No, they are not lying to you, and yes, if the difference in rolling diameter is too great front to rear the DSC lights will come on.

A rule of thumb is to be within a few revolutions per mile of difference. More than that stresses the transfer case. If the rears have more than 50% tread depth remaining you should be fine.

Discount may not be 'lying' but they are generally dumb and/or indoctrinated by corporate to sell tires.

This myth of 'damage' and 'dsc' has been around for a while. Lets debunk it, shall we?

Take a stock size tire- 255/50-19. Diameter is 29.1, 715 revs per mile and 10/32 tread depth.

Tire is done at 2/32. So 8/32 of tread represents 100% of treadwear.

4/32 is 50%..in the OPs case lets assume that his "15000 more miles" is, indeed, 50%

The question then become this:

If the fronts are at 100%, the the rears are at 50% will this cause problems? The rears will be 4/32 less...or 1/8" or 0.125

The delta in revs per mile is approximately 3-4 revs per mile, or 3-4 revs per minute at 60mph..insignificant for a modern differential. IMO

BMW OE tires, in the staggered set up, allow (as I recall) 0.1 inch of mismatch- and that is BEFORE you consider that BMW specs a (significantly) higher inflation pressure on the rear wheels.

SImply stated the BMW differential can easily accommodate these minute differences in tread depth as they are much SMALLER than the variability in the tire size that result from using the BMW specified sizes and inflation pressures.

BTW, I hear this crap all the time at tire shops- and the owner who is a friend, just shrugs and says "hey, sells tires"

A

[JCL]

I would use 4 revs per mile as a guideline. That is pretty close to the 50% of tread wear I suggested.

I have read 3% as a BMW recommendation for the maximum difference in revs/ mile.

I have read reports of x3 transfer case issues with far less than that. DSC lights and TC failures. It is the same X-drive design, but perhaps the tolerances are less.

Volkswagen allow 2/32" tread difference on their AWD models.

No argument on whether Discount are habitual liars or not, but even a stopped clock is right twice per day. AWD systems deserve tires with similar rolling diameters. Especially since transfer cases cost more than tires.

I just bought new tires. Four of them.

[JCL]

Originally Posted by ard

If the fronts are at 100%, the the rears are at 50% will this cause problems? The rears will be 4/32 less...or 1/8" or 0.125

That .125 is radius. Multiply by 2 for diameter. That is 2.5 times the BMW diameter limit difference of .1" you reference.