I resently made trip to wash. state then to calif.and back to tx. in my 02 4x4 5.4. I average 18 -18.5 mpg. one time 20.2. I normaly get 15-16 on I-10 back and forth to Louisiana. I could not believe I got that good at higher elevation. Sounds like engine running to lean down here in tx. and higher elev. it's just right, but computer should adjust for the change in alt. shouldn't it. Any sugestions.Someone said to have high alt. WHY!