Was I thinking clearly? X6 M tire purchase

[ToadStool]

I'm new here and this is my first post and perhaps this has already been discussed and if so, I apologize.

I recently took possession of a low miles 2018 X6 M. With winter weather around the corner and the original summer performance tires seeing better days, a few weeks ago I purchased new all-season tires.

I was a bit surprised to see the stock rear tire size 325/30R21 and front stock tire size 285/35R21. My very limited research (tiresize.com) shows that the rear tires are 28.7" in diameter while the front tires are 28.9" in diameter. This in turn leads to 704 revolutions per mile for the rear while the front tires will rotate 699 revolutions per mile or 5 less revolutions.

If we take it to an extreme say 100 miles of travel, then we have the rears circling 70400 times while the fronts are circling 69900 times - or 500 fewer revolutions. That seems like a lot of potential friction and I'm guessing something has to be wearing out a bit prematurely to compensate for this not-so-good use of energy.

I always thought the generally historic rule for any 4WD/AWD is that all tires ought to be the same diameter. Though my understanding is that BMW has generated a means to compensate or allow for slightly different tire diameters.

Anyway, when I purchased new tires I kept the rear tires the same size at 325/30R21 but I altered the front tire size to 275/35R21 - 10mm more narrow than stock. As a result, while the rear tire diameter remains at 28.7" the new front tires are 28.6" - cutting the stock tire config difference in half from the original front config.

Now the front tires should rotate 706 revolutions per mile while the rears remain at 704 revolutions per mile. Or only 250 revolutions difference over 100 miles. Doesn't that half the amount of friction potential of the stock config?

By doing this, did I lessen any friction and thereby potentially improve mpg and/or drivetrain longevity? Or does it really matter? I mean, everything has a shelf live, right? And in the end aren't all improvements of any sort the result of lowering resistance and/or improving efficiencies?

Obviously, BMW knows what it's doing but I kinda' cringed a bit when I saw the tire size difference between the stock rear and stock front tire sizes.

My thoughts are, no matter how much any mfg'er compensates for slightly different tire sizes for AWD vehicles, my vehicle should always be better off with less potential friction the more identical all tire diameters are.

Or am I just overthinking things a bit?

[TTV8M]

I like the math

[cmyx6go]

You're posting in the wrong forum for a 2018.

Post here, there is a lot more traffic. https://f15.bimmerpost.com/forums/fo...play.php?f=509