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      11-11-2013, 02:22 PM   #1
Superfly_M3
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Winter Tire Question

I currently have OEM BMW winter package setup:
Style 270, 18x8 ET 20 with 235/40/18 Michelins Pilot Alpins. No spacers. Oddly, there is no threadwear indicator on the OEM tires

The speedometer is affected by "a lot" at 135km/h indicated on the speedo, GPS confirms the speed to be at 125km/h. The tire diameter is 25.40"

Original front 245/35/19 are 25.47" and rears 265/35/19 are 26.30"

My back tires are "gone" and need replacements. I was wondering if anyone ran 245/45/18 in the back (26.68") and 245/40/18 in the front (25.71")
Because my car is lowered, the extra clearance is a good thing in the winter.
Can the 18x8 rim fit a 245 series tire?

Thanks in advance
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      11-11-2013, 03:05 PM   #2
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Yes. 245/40/18 is what I ran the last 2 winters on an 18x8 square set-up. This year I'm running OEM 219's (8.5F, 9.5R) with 255/40/18 square tire set up. You'll be fine.

EDIT: Don't run a 245/45. Lol.
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      11-11-2013, 03:42 PM   #3
Superfly_M3
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Thanks for the reply,
Why no 245/45/18?
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      11-12-2013, 05:22 PM   #4
ben@tirerack
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245/45R18 fits an 18x8 wheel, and would probably be fine on the rear.

But it is a lot easier to have all 4 tires the same. You will have more tire choices because you don't have to hunt for matching front/rear pairs. You will be able to rotate so that they wear out at the same time.

Regarding ground clearance: I doubt the extra fraction of an inch of ground clearance at the rear will give you any realistic benefit in deep snow. If you've already pushed your front aero through the snow, you're not likely to get the rear end hung up.

Regarding speedometer, the factory calibration is a big part of the reason you are noticing so much difference. Looking at some math:

26.3/25.4 = 1.035 ~3.5% error from tires
135/125 = 1.08 ~8% error observed.

Tire error is 3.5%, speedo error is 8% Just over half of the error is built in.
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      11-19-2013, 12:13 PM   #5
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I like "square" snow setups. The tires are soft and the rears wear crazy fast, especially if you live somewhere that the temps bounce and you get a few warm days every winter month. With squares you can rotate.
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