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Originally Posted by 007X3
I took that into consideration when buying Pirellis. The 245/45/18 front, 275/40/18 rears are identical 26.7" in diameter and identical 780.4 revolutions per mile. Still the intermittent X-drive problem (noise) persists. All I can think of is that the tires are rotation at slightly higher rpm. About 2.7% higher that the car thinks they should be rotating.
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The point I don't get is the assumption that the car thinks the tires should be going a certain rotational speed. The only way the car knows how fast the tires are going is by the wheel speed sensors. It doesn't have another reference signal, relating engine speed to true ground speed. There is usually a torque converter in the equation, so it isn't possible to just compare engine speed to driveline speed. You could have an issue with the torque converter locking up, separate from any problem in the transfer case.
The speed difference between different tires could absolutely cause a problem in the transfer case. I know of no factory or dealer programming for that, but I don't work for the factory or a dealer, so maybe it has just never come up.
I don't think the small differences you are quoting front/rear for rotational speed should cause a problem. You are assuming that the actual rolling diameter is as published by the tire manufacturers, and that may not be a good assumption.
If you want to check, it is very easy to drop the tires on one axle by 5 psi, test drive the car, then put those tires back to spec and drop the tire pressure on the other axle by 5 psi. Don't drive right to the handling limits when doing this, but see if the noise is gone in a straight line. You can easily achieve a different effective rolling diameter just by adjusting tire pressures. If the noise is worse in one direction, and goes away in the other direction, you have found the source of the problem.
It might just be me, but I would just put the stock tires back on and enjoy the car.