Originally Posted by jdstrickland

Your Sport should have 19" wheels on the ground, and a 17 in the trunk. All of the X5's that we looked at had the same 17" rim in the trunk, no matter what wheels were on the ground. The tire in the trunk should be a 235/65x17. An equivalent 19 would be a 235/55x19, although the larger rim should also be wider, and I am not sure what they use.

You said your X5 is the 3.0 with a sports package, it should have 19 on it. The pic looks like 17s.

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You can easily calculate the overall diameter by multiplying the width x the aspect ratio (235 x .65) to get the sidewall height in millimeters. Multiply this result by 2, then divide by 25.4 to convert the sidewall to inches, then add the rim size. The result is the overall diameter.

235 x .65 = 152.75 x 2 = 305.5 / 25.4 = 12.02 + 17 = 29.02. All X5s should have a tire that is close to 29 inches in diameter. As the rim gets larger, the aspect ratio (65, in this example) gets smaller to give a smaller sidewall, leaving you with the same overall diameter. If the rim gets larger AND the width is increased, then the aspect ratio is reduced some more so that the overall diameter does not become larger.

The 235/65x17 becomes a 235/55x19, with an overall diameter of 29.17. I'm not sure, but I would expect a 19 to have perhaps a 255, so it would be a 255/50x19, or a 265/50x19, to give a diameter of 29.03 to 29.4, respectively.

There are staggered fitments available, too. These fitments have a wider tire on the back than is on the front. For example, the rear might be something like a 315/40x19, to give an overall diameter of 28.92. A 325/40x19 has a diameter of 29.23 inches.

BMW will select front and rear tire sizes that will be within about 1/4" of being the same from front to rear. In this example, the 255/50 (29.03") on the front would be coupled with the 315/40 (28.92") on the rear, and the difference in diameters is .11 inches. The difference in circumference is 0.3456", which is meaningless.