Here's an idea that might get you a relatively cheap data point before going through the full swap to see what you get ...

Put on a set of wheels/tires that have a 5% (or whatever exactly you're aiming for) smaller diameter, and take some test runs on those. I can guarantee swapping tires is easier than swapping differentials. And the results will probably be more relevant and accurate that anything you could get using a formula.

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2001 X5 3.0i, 203k miles, AT, owned since 2014