[Young04]

I am thinking of getting the M550i to replace my A6. Love the 20 inch 668M wheels (and do not really like the 19s).

I do have a concern about tire/rim damage. Any issues going to:

265/35/20
295/30/20

Anyone do this? Fitment issues? Rubbing? Trying to get a little more sidewall. The front to back ratio stays roughly the same and the speedo will only be a little off.

[VirtualGuitars]

Originally Posted by Young04

I am thinking of getting the M550i to replace my A6. Love the 20 inch 668M wheels (and do not really like the 19s).

I do have a concern about tire/rim damage. Any issues going to:

265/35/20
295/30/20

Anyone do this? Fitment issues? Rubbing? Trying to get a little more sidewall. The front to back ratio stays roughly the same and the speedo will only be a little off.

The 668M are beautiful. I have the other wheels, but I'm not sure I'll ever buy an M550i with 20 " wheels again. They look fantastic, but it's a very rough ride and potholes are absolute killers. One of my rims bent just slightly and the tire bubbled up in five places.

[Deda]

I will strongly recomend NON RFT....about size you should be worried about rubbing rear tire on fender when you drive 3 persons on rear seats.

[Young04]

Originally Posted by Deda

I will strongly recomend NON RFT....about size you should be worried about rubbing rear tire on fender when you drive 3 persons on rear seats.

I'm now thinking that 245/40/20 and 265/35/20 may be the better option. Speedo will be off a little but the tire diameter ratio stays the closest this way. Will go with non-RFT.

[jjscsix]

Call Discount Tire and ask them. I went to 285 and 255 in 19” wheels and they told me they could absolutely confirm they would definitely fit with no issues, and I went to a 1/2 inch wider 7 series wheel. Obviously mine were different series, but the point is that they have a good data base of known fitments

[Superjackson]

Widest u can fit on 668 is 245 front, 275 rear. Tallest you can fit is 245/40/20 and 275/35/20.
If you want 265 and 295, you will need a 9" wide wheel in front and at least 10" wide wheel in rear. To prevent rubbing on 265/35/20, you need between 25-33(preferably 28-30)offset front. The rear has very much space to fit 295/30/20, any offset between 35-45 will be safe from rubbing even if lowered.

[Young04]

Originally Posted by Superjackson

Widest u can fit on 668 is 245 front, 275 rear. Tallest you can fit is 245/40/20 and 275/35/20.
If you want 265 and 295, you will need a 9" wide wheel in front and at least 10" wide wheel in rear. To prevent rubbing on 265/35/20, you need between 25-33(preferably 28-30)offset front. The rear has very much space to fit 295/30/20, any offset between 35-45 will be safe from rubbing even if lowered.

Thank you. Noob with this stuff. If I understood you correctly, 245/40/20 front and 265/35/20 rear will be fine with no offset?

[Superjackson]

Originally Posted by Young04

Thank you. Noob with this stuff. If I understood you correctly, 245/40/20 front and 265/35/20 rear will be fine with no offset?

It will be better to use 275/35/20 instead of 265/35/20 to better match the height of 245/40/20 for the xdrive system. 265/35/20 is 1.5% shorter than 245/40/20, it is just barely okay but it's best to keep them within 1% of the same height if you don't want service people blaming whatever problems you have on the tires even if they weren't the cause.

[hlothery]

I have the 668Ms, 20 inch. I have had no problem since switching to Michelin PS3ZP runflats. Very smooth ride, much more grip, and have had no further sidewall damage (popped two of the Goodyear F1 Asymmetrics sidewalls which were OEM, without rim damage). Didn't change tire size, just bought better tires.

[Young04]

Originally Posted by Superjackson

It will be better to use 275/35/20 instead of 265/35/20 to better match the height of 245/40/20 for the xdrive system. 265/35/20 is 1.5% shorter than 245/40/20, it is just barely okay but it's best to keep them within 1% of the same height if you don't want service people blaming whatever problems you have on the tires even if they weren't the cause.

My thinking was that the diameter ratio of front tire to back tire stays closer to stock with the 265/35 in the back.

So stock tire diameters are:

245/35/20 = 26.8
275/30/20 = 26.5

Ratio is 1 : 1.01132

With my proposed sizes, the tire diameters are:

245/40/20 = 27.7
265/35/20 = 27.3

Ratio is 1 : 1.01465

With 275/35/20 (diameter = 27.6), that ratio goes to 1 : 1.00362

So the ratio with 265/35/20 is closer to stock.

Another way to look at it is that with stock tires, the fronts are .3 inches bigger in diameter. With 245/40/20 fronts and 265/35/20 rears, the fronts are .4 inches bigger in diameter but if I went with 275/35/20, the fronts would only be .1 inch bigger in diameter.

Am I going about it incorrectly?

[Superjackson]

Originally Posted by Young04

My thinking was that the diameter ratio of front tire to back tire stays closer to stock with the 265/35 in the back.

So stock tire diameters are:

245/35/20 = 26.8
275/30/20 = 26.5

Ratio is 1 : 1.01132

With my proposed sizes, the tire diameters are:

245/40/20 = 27.7
265/35/20 = 27.3

Ratio is 1 : 1.01465

With 275/35/20 (diameter = 27.6), that ratio goes to 1 : 1.00362

So the ratio with 265/35/20 is closer to stock.

Another way to look at it is that with stock tires, the fronts are .3 inches bigger in diameter. With 245/40/20 fronts and 265/35/20 rears, the fronts are .4 inches bigger in diameter but if I went with 275/35/20, the fronts would only be .1 inch bigger in diameter.

Am I going about it incorrectly?

The stock setup is not ideal as well, it's just a compromise based on whatever factors like runflat tire availability, wheel width, and whatever else. The ideal setup is having the front and back tires making exactly the same revolutions with a square setup, but that is not possible on a staggered setup so the best would be having them be as close in diameter as possible.

[Young04]

Originally Posted by Superjackson

The stock setup is not ideal as well, it's just a compromise based on whatever factors like runflat tire availability, wheel width, and whatever else. The ideal setup is having the front and back tires making exactly the same revolutions with a square setup, but that is not possible on a staggered setup so the best would be having them be as close in diameter as possible.

I thought that the car's systems accommodated the difference in wheel sizes and that that ratio should be kept.

[jjscsix]

Originally Posted by Young04

I thought that the car's systems accommodated the difference in wheel sizes and that that ratio should be kept.

You are of course correct. I feel 100% positive that BMW engineers are smart enough to figure that out

[Young04]

Originally Posted by jjscsix

You are of course correct. I feel 100% positive that BMW engineers are smart enough to figure that out

My point is:

Which is correct - maintain front/rear wheel diameter ratio or go for square all around like Superjackson says?

[NYCBMWFAN]

Originally Posted by Young04

My point is:

Which is correct - maintain front/rear wheel diameter ratio or go for square all around like Superjackson says?

I upgraded sidewall when I got my car immediately to the sizes you are inquiring about. 245-40-20 F; 275-35-20 R with no issues. I went with Conti DWS06 NRFT. It's a very nice ride and I live in NYC area. Do it!

[jjscsix]

Originally Posted by Young04

My point is:

Which is correct - maintain front/rear wheel diameter ratio or go for square all around like Superjackson says?

I doubt if the minor difference in ratio of a "square" set would matter, but I would think that staying at the same ratio as what came from the factory would be the more important factor.

But with today's complex ECUs, who knows.

[KenM550i]

Originally Posted by NYCBMWFAN

I upgraded sidewall when I got my car immediately to the sizes you are inquiring about. 245-40-20 F; 275-35-20 R with no issues. I went with Conti DWS06 NRFT. It's a very nice ride and I live in NYC area. Do it!

You have a picture?