[MjOF528]

It is somewhat confusing to read about the 1% "allowed" difference in size but at the same time others list all kinds of different configurations which don't fall in that category. My dealer won't give any detailed info since they mainly interested in selling the tires they have. I would appreciate some advice in regards how critical a change from stock tire 225/55R17 (Dia. 26.8 in.) to 235/45ZR17 (Dia. 25.5 in.) or 245/45ZR17 (Dia. 25.8 in.) would be.

[mejori]

Originally Posted by MjOF528

It is somewhat confusing to read about the 1% "allowed" difference in size but at the same time others list all kinds of different configurations which don't fall in that category. My dealer won't give any detailed info since they mainly interested in selling the tires they have. I would appreciate some advice in regards how critical a change from stock tire 225/55R17 (Dia. 26.8 in.) to 235/45ZR17 (Dia. 25.5 in.) or 245/45ZR17 (Dia. 25.8 in.) would be.

Dealers do not like to stray from "factory sizes" for liability reasons.

As far as the 1% I believe it has to do with a staggered setup
(a 1% rolling difference between the front and the rear).

I am pretty sure it is fine to use either of those sizes all the way around because the rolling size will be the same and not cause DSC/DTS lights to come on.

hope this helps..maybe someone with direct knowledge will chime in.

[Tallest]

ˇ1% stock versus fitted and 1% front versus rear are different things.

So, for xdrive you want to have same rolling diameter all around to make sure this fragile transfer casing isnt getting overwhelmed. Go wit sameish rolling diameter all across 4 tyres, but avoid staggered setup on xdrive. BMW has it on X5s and X6s, but NOT on 5 series, unless it is a 550md xdrive. There the setup is staggered on 20in rims only. But I bet if you looked up tyres fitted to the m550d xdrive you will see they are very close in diameter and within margin of error. But on 19s its already a square setup. Sorry hope this helps.

[TEF]

Aside from tire size, each size comes with a load rating. If for example the stock size has a load rating (and these are available on Tirerack for every tire sold) of, say "97", then any replacement tire (no matter the size, should have a load rating of 97 or greater. The load rating ensures that a tire is designed for the weight of a specific car. In other words, the tire used for a 3 series sedan, is not only smaller in size, but usually has a lesser load rating.

Therefore, one can pick a choose tire sizes (provided all four match in diameter and physically fit the wheel wheel) as long as they are of sufficient load rating for that vehicle.

Often people substitute 17" wheels and winter tires for 18's, but they are of the same diameter and load rating.

[Techgeek80s]

Originally Posted by MjOF528

It is somewhat confusing to read about the 1% "allowed" difference in size but at the same time others list all kinds of different configurations which don't fall in that category. My dealer won't give any detailed info since they mainly interested in selling the tires they have. I would appreciate some advice in regards how critical a change from stock tire 225/55R17 (Dia. 26.8 in.) to 235/45ZR17 (Dia. 25.5 in.) or 245/45ZR17 (Dia. 25.8 in.) would be.

Been dealing with this for like 6 months and still haven’t pulled the trigger. Don’t know about you but I wanna rotate my tires cause if not yiur gonna be spending hella money on tires. Even with that 1% tires don’t wear evenly and me personally I don’t wanna ruin my tranny. I know 20x10 fit in the front with no fender modification and 20x11 fit in the back with no mods either tires size wellll..... I’m here tying to learn that to be honest but I was gonna 20x10 all the way around with spacers in the back but not sure I wanna do that either. This will be my lost all wheel drive car cause this is a pain to deal with

[Noggie]

I have a M550D X-drive and due to a tire failure on a vacation trip last year, I had to replaced my front tires with some cheap ones, which was all I could get my hands on at the time and place.
This was the stock square 245/40 19" tires.
My rears are probably 50% worn, and I have had no issues what so ever with that difference in tire wear, the rears are 6mm / 1/4" smaller in diameter than the fronts.

I will go for a staggered 20" setup this year, so I'm not bothered about this.
Those 19" stock wheels may end up as winter tires.

BMW lists 245/35-20" and 275/30-20" as a staggered tire option for my car.

If you stray away from the stock diameter you speedometer accuracy will suffer, too small diameter and your indicated speed will be higher than actual, a larger diameter will potentially give you a higher actual speed than indicated.
Stock the BMW is 2kph/1mph lower than actual speed. This is my experience from speed traps and GPS with stock tires.