Quote:
Originally Posted by Surly73
Be very careful.
"Tire size calculators" use nominal sizes for the numbers you enter. There is a wider-than-you-would-like acceptable range of tolerance around those nominal sizes. Worse, if the tire manufacturer doesn't consider the sizes you've picked as a "set" for staggered use, the two sizes could be at opposite ends of the tolerance range.
If you can get "star" rated tires or similar by choosing popular BMW staggered pairings, they are made so that rolling circumference is within tolerance for X-Drive.
It wasn't in an AWD application but I've "been there, done that" and despite thumbs up from tire size calculators and other owners with a size combination on a past car, it looked odd because of opposing tolerances. At least there was no transfer case to destroy in that case.
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Thanks for being direct, personal opinions are too subjective so I'm looking for hard facts.
It sounds like if I go with the 255 fronts (nominal diameter difference of 1.1%) I'll be well outside the safe range for xdrive as the rears wear and the difference grows. As you said, if the fronts and rears are at opposite ends of the tolerance range the difference would be even greater.
It sounds like Tirerack's recommendation (What I'm running now) is my best option:
Front: 245/35ZR20 (26.8” diameter, 777 revs/mile)
Rear: 285/30ZR20 (26.8”, 777 revs/mile)