you're not going to noticeably change the size of the tire by changing the pressures and even if you could it wouldn't remain constant as the tires heat up and cool down at different rates.

Try to find what cold pressure most have had good luck with and then watch for tire wear patterns to adjust as needed.

overall tire size can be different between brands and even models. The big reason to not mix and match tires all over the place is because of this difference. since the front and rear diffs are both open (from what I understand) I would only worry about matching tires on an axle.

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2006 Audi A3 2.0t DSG apr stage 2
2002 BMW X5 4.4
1997 BMW 328ic sport. moreagrun metallic
1989 BMW 325is with / M50b25nv swap