So I have a '99 QX4, and like all of you the fuel mileage is dismal. So, I am curious what people have done to improve the overall power, fuel mileage, and breathability of the engine?

I know some have added some fuel mixtures that have improved the mileage an extra 50 or so miles per tank full, but I am curious about aftermarket air intakes, free flowing mufflers, snorkel kits, chip sets, anything that will improve the overall mileage and power of the engine. I know those of us at higher altitudes all get a dismal 12-16 miles per gallon, but someone must have played around with things a little?

Since I live in Colorado and use my vehicle up in the 12,000-14,000 feet above sea level range for my job, I can use any additional engine efficiency possible.

Any ideas would be greatly appreciated! Thank you for your time!