Best mileage that I've seen in my 05 Xterra has been at altitude. It was especially noticable above 8,000 ft. I attributed it to thinner air, thus, less wind resistance. So a chip won't do any good in my opinion. I agree with the others who said that slowing down is the best option to increase fuel economy. When I switched to All Terrain BF Goodrich tires my mileage went down 1 to 2 mpg due to the more aggressive tread. I track mileage closely during our annual vacation from Toronto to Utah, New Mexico, California, etc. 30 or more tanks of gas in 3 to 4 weeks gives me a pretty good idea how various modifications or changes in driving habits impact driving. I should mention that using premium gas showed no improvement in fuel economy. Different air filters also showed no difference. Cruise helps but not as much as slowing down.