Tire help...

[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 Unpublished.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...