18 inch tire size question

[rjbaren]

I have a nice set of 6, 18 inch M parallels. I am lucky in that I can go staggered or square and I have decided to go square. The question is size. The OEM 235 50 16s I calculate overall as 641.4 millimeters. Tire rack recommends 245 40 18 and I calculate the overall size in millimeters is 653.2. But, I also calculate 235 40 18 overall as 645.2 and this seems the closest to the OEM size when using an 18 inch wheel and a square set of tires. Am I missing something, does anybody understand why Tire rack recommends the 245 40 18? What are your guy's opinions because most of you know a lot more than I do?

[Carl1011]

When I purchased my 840 it came with 245 / 40 / 18 with hub rings 74.1 x 72,56mm.

[rjbaren]

I think your wheels are for an e39.

[Dr. T]

The matching diameter to the original diameter would be 235/40/18. I don’t know where Tirerack gets their info/recommendation from.

That said, for some reason the BMW CSI recommendation for the front M-Parallel is 245/40/18. This results in a taller tire overall. Perhaps they wanted more sidewall in the 18”...with the e38 being even taller...

So I guess both sizes work fine.

[Timm]

Yes, CSI tyres are 245.40 ZR18 and 285/35 ZR18:



More info here:

http://www.meeknet.co.uk/e31/Timms_b..._tyre_page.htm

[rjbaren]

Thank you Timm. Per the chart, and the 8" width M Parallels the 235 40 18s are correct for the square set. So the best part is the 235 40s are a little less expensive and, the Hankook Noble S1 Noble 2s in this size weigh only 22 lbs. There is no weight penalty between my 16 inch OEM turbines with BF Goodrich Comp 2 AS (45.9 lbs.)and the proposed square 18 x 8 inch M Parallels with Hankook 235 40 18s (45.6 lbs.). Thank you for the information.

[Dr. T]

Thank you Timm. Per the chart, and the 8" width M Parallels the 235 40 18s are correct for the square set. So the best part is the 235 40s are a little less expensive and, the Hankook Noble S1 Noble 2s in this size weigh only 22 lbs. There is no weight penalty between my 16 inch OEM turbines with BF Goodrich Comp 2 AS (45.9 lbs.)and the proposed square 18 x 8 inch M Parallels with Hankook 235 40 18s (45.6 lbs.). Thank you for the information.

You should re-check the math. I find it hard to believe that the 16” option will be the same weight as 18”. The weight of the rubber is somewhat negligible compared to the added weight of an 18” wheel. Maybe 5-10lbs. heavier.

Not all that sure that the Tirerack specs on tire weight are all that accurate either. However, a quick check of 2 different tires shows the 245/50/16 (235 not available) being ~1lb. lighter than a 235/40/18 of the same brand. The 18” are surely more than 1lb. heavier than the 16”’s.


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[David9999999]

As Tim said, the OEM size for an 850CSi is 245/40-18 front and 285/35-18 rear. This is what I'm running on my car that I've upgraded to staggered M-Parallels, and I think the setup looks and handles very well. I wouldn't want to downsize the diameter of the tires compared to OEM size.

[rjbaren]

Style 9 Turbines weighs in at 17.7 lbs. plus .2 lbs. for the hubcap. BF Goodrich G Force Comp 2 All Seasons weigh in at 28 lbs. totaling 45.9 lbs. per corner.
Style 37s weigh in at 23.6 hub caps are negligible. Hankook Ventus S1 Noble 2 All Seasons weigh in at 22lbs. totaling 45.6 lbs. per corner.
Wheel weights come from BMW Parts catalog. Tire weights come from Tire Rack. That's all I have to go by till I weigh them myself. But it's looks promising, and this is why I want to do the square set and use the 235 40 18 Hankooks. If this is correct, I can have my cake and drop 1.2 lbs. in the process.