Tire help...

[mpower1]

Hey i searched and could not find a clear answer and was hoping someone would help me out on this... I'm looking to purchase a set of 295/30/21 Dunlops for the front of my 2008 X5. Does anyone know if these will fit without rubbing issues... Thanks.

[lanbrown]

How is the tire circumference of these tires to the ones on the back? That is something else you need to check.

[mpower1]

I'm running factory sizes. 325/30/21 in the rears. Stock fronts 285/35/21.

[ard]

Originally Posted by mpower1

Hey i searched and could not find a clear answer and was hoping someone would help me out on this... I'm looking to purchase a set of 295/30/21 Dunlops for the front of my 2008 X5. Does anyone know if these will fit without rubbing issues... Thanks.

Originally Posted by mpower1

I'm running factory sizes. 325/30/21 in the rears. Stock fronts 285/35/21.

You will damage the differential.

Do you know what you are doing??why would you propose mismatched tire diameters on an AWD car.

Factory sizes are the same outer diameter, within 0.1".... 295/30 and 325/30 are not

[mpower1]

Hence the title tire "help"... Thanks for the info.

[Mongojoe]

You want to stay within 3% of the OEM diameter size to avoid significant issues, per this tire calculator (enter in stock size, then the alternate size(s) you are considering):
Tire Size Calculator - Tire & Wheel Plus Sizing

I'm trying to do some wicked new math trying to find all-season rubber for the X5 staggered-width 19's.

[ard]

3% is a nonsense number.

Look, the issue is TIRE ROTATION and the DIFFERENTIAL. If one axle rotates faster (or slower) than another axle, the differential need to match. TOO MUCH difference and the diff overheats and is destroyed.

So the question: what is the allowable 'mismatch'? What it the BMW 'spec'?

WE DO NOT KNOW. Do Not Know. Unpublished.

You can read every forum in the world and find lots of authoritative posts, but it is not published.

So how do we proceed?

We look at Stock, OE tire sizes- and specifically we look at the mismatch in 'revs per mile' to see what BMW ships.


For example:

285/35-21 721 revs/mile
325/30-21 725 revs/mile

So a 4 rev difference.

Imagine driving at 60mph. in one minute you cover one mile. In that one mile the differential rotates 4 times. Pretty minimal.

4/725 is 0.55% Yes, less than 1%

3% is SIX TIMES what BMW ships.

Notes:
This is an example with ONE tire model. Each tire model will be different.


Car loading, inflation pressure and tire heat will ALSO change individual tire pressures. (So in the above example, the spec says 721 and 725.. but due to pressure differences there will be a + / - around these...obviously there is a BMW spec that is a "nominal" (ie 721/725) along with a tolerance... probably +/- 5-10? The key, however is that 'tolerance' that BMW needs around whatevr the nominal they chose will apply to us/you and any tire you select from tire rack. The tire you buy will- when you dirve- have the same pressure fluctuations and loading variations.

So you cant say "BMW allows 4, but we know it can be as much as 4+5+5=14...so I will select 14 as my nominal"

Anyway, conservative for sure- but I get annoyed when people blithely declare some % they pull out of the air.

Someone want to look up every OE tire model, size and ship configuration? list the revs/mile for each? It would tell you where BMW design nominal has been set...

[lanbrown]

Just to add to what ARD has stated, if you replace just the front or the rear, there will be a difference between the front and rear as well. The system can cope with this, but the OP was also changing the front to essentially a different size.

So when the OP stated he was going to replace the front, that is why I suggested to them to check the tire circumference. If you are only changing one axle, you cannot be too far off from what the factory/previous tires are.

[mpower1]

So how is this different from adding spacers on just one axle...? Wouldn't that change rotation also...? Example... Putting 15mm spacers on just the rear axle.

[ard]

Originally Posted by mpower1

So how is this different from adding spacers on just one axle...? Wouldn't that change rotation also...? Example... Putting 15mm spacers on just the rear axle.

Of course not. The spacer simply moves the wheel along the axis of rotation. You could have the wheel 10 FEET outside of the body and it still rotates according to its diameter.