Over the last week I have noticed a drop in my MPG. My last road trip I was able to get 22 MPH on the freeway out of my 05 yukon xl. This trip I only got 17.9 mpg. I drove the same route, same speed, with the same weight. Tires are inflated to the same PSI. I have not added any mods to the truck since the last trip. I checked the plugs and they are new. The wires are new. I cleaned the MAF sensor and throttle body. There is not a changeable fuel filter. It idles and runs smooth. I can't complain to much as I a still getting 16mpg in the city, which is well over the epa rated 13 mpg. But, I still want my highway mpg back. Today I will connect my programmer and run the monitor to see what my O2 sensors are reading and if anything looks out of the ordinary with those. Any ideas about what would cause an instant decrease would be appreciated. I checked O2 sensor readings with my monitor while I drove. Both banks had the same reading. I did notice that the throttle percentage never went to 0, like my 02 did. It was always at 11% even with my foot off the gas and with the truck off. It does not have a tps like my 02 did. The hole were it was on my 02 is plugged on this truck.