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I have a 15' sDrive28i. ~25MPG mostly highway at 80MPH.
The economy figures are calculated from air resistance, transmission loss, and rolling resistance and sustaining that load in a lab environment where the turbo hardly ever kicks in.
At 70MPH, the X1 has a drag of 470 newtons. At 45MPH, the X1 has a drag of 200 newtons. A gallon of gas has 130 mega joule which at 200 newtons, will last you about 400 miles. You take out ~80% loss via heat and within the engine (since the engine is not in the optimal efficiency range 3000-4000 RPM) and 5-10% loss between the engine and the ground. That gets you to about 50MPG which is roughly what you'll get at 45MPH. At 70MPH, some of the static losses do not scale linearly so you save some but lose out since you're expending more than twice the energy on air resistance.
Also to consider, the turbo kicks in a lot since the N20 has a small displacement and low compression ratio.
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'15 X1 sDrive28i
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