I noticed that if I fill up at about a .5 tank I get about 18mpg, but if I let it get to empty I only get about 16mpg. Those are highway miles with AC and CC. Plus i have the Airraid intake and a Flowmaster 50 SUV muffler.

Do you want better gas milage? Are you sick of the gas price ? Do you think Xterra is a gas gasler ?
If you answered yes to any of this questions and dont know what to do ,well let me give you an edvise .buy a Prius and forget about the mpg. If you like xterras
well that overwrites the shity mpg.

Something to ponder...I've always used a good grade of gas in my x....BP..Texaco..Exxon..most always the gas mileage has been consistent..17C.19H...sucks, but it is what it is...
I'm at Kroger one night...the guy says you can save 10cents gallon because you have spent $100 or some rot...This was a few months back when prices were doing the creep up thing. and everyone was just thinking the prices were bad...anyway..I thought 10cents..what the hell..so Im filled 'er up..it seemed to run ok no more pinging and knocking than normal...so I used it (Kroger gas) again..I happen to check my mileage after this time and it had fallen to 14 mpg..I didn't think about the gas i was using and was preparing to go through the tune-up deal (it only has about 53k miles on it)and went back to the BP gas...immediately the gas mileage went back to 17...Daddy told me along time ago that you get what you pay for...

The gas for all the different gas stations in the area usually comes from only a few refineries. In other words, the gas you may have used at Kroeger(sp?) and BP and such might have been from the same refinery anyways. I have not seen any difference in mileage from gas station to gas station.
My Low Fuel light comes on like clock work + - 5 miles on the odometer.

you know this because???....your neighbors ex-brother-in-law used to drive a tanker delivery truck to the local 7-11???

He's right.

I used to work for BP and lots of local stations bought gas from our rack. The difference is they are buying the raw gas w/o additives. When I worked for BP our formulation was the most expensive to produce. Not sure if that is true anymore I left right after the Amoco/BP merger.

Later
JimC.

I have a 2004 4WD V-6 X. I get around 21.5 mpg on the highway and about 17-18 in the city. I drive at or right above the speed limit. I was really anal about breaking the engine in when I bought my X. Maybe that's why I'm getting such good mileage...Does anyone have a thought on this? I also have a K&N filter and I keep my tires at 42 psi.

I think (IMHO) that the tire pressure, engine break-in and then the K&N help fuel economy in that order. I don't think the K&N helps until about 2500 rpm or higher. At 70 in my 05 I run at 2000 rpm. So no help from a K&N.

Drive conservatively, keep your tires inflated and reduce air drag and weight of your vehicle will optimise your fuel economy. Intake and exhaust add-ons don't help that much.