Well, I think we could be a little more helpful. Just my opinion.

1) No aftermarket part is going to improve mileage measurably. Save your money. If the vehicle is in a reasonable state of tune, then that is it. If it needs a tune, then plugs and a clean air filter may help. Everything else is a hoax. That particularly applies to everything sold on ebay.

2) Correct tire pressure is important. Remove racks and other items that impede airflow.

3) If 89 doesn't drive differently, use it. If you can tell the difference, go back to 91. You can't hurt the engine with 89 if the fuel is clean. Remember that your 89 isn't necessarily the same spec as purchased by those who see a power reduction with the fuel they buy locally. Fuel varies widely across the country, which is why BMW specifies 91, to cover the variability. My best mileage ever, measured over a full tank, was on 89.

4) Now the part that matters. It is all about how you drive it. Drive ahead (watch what is coming up, and anticipate), avoid hard braking, avoid jack rabbit starts, keep to a moderate cruising speed, avoid warming the engine for more than 15 seconds before driving moderately until it is fully warmed up.

5) Finally, drive it less. Combine short trips for errands. Telecomute one day a week if you can. Ride a bike if the trip is under one mile.

I knew the mileage of my X5 when I bought it. That doesn't mean that I want to waste fuel or money. Those who don't think that they can improve their mileage by monitoring their driving habits can skip the above point 4.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue