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The fuel management system in your vehicle adjusts to altitude very, very quickly-less than a second.I have heard There is another sort of system in some vehicles that does adj to your driving style,and this is said to take several days/rides(I have my doubts about how much mpg difference this makes-I have heard of it in reference to some Hondas).
Bottom line,I suspect your mpg difference might be altitude or the prevailing winds on your trip to California(Cali?).I drive from New Orleans to Flagstaff AZ fairly frequently. I get better mpg-much better-once I get to 3000-4000 feet.There is much less air resistance at altitude,so you get better mpg.In the old days-with carbs-a sea level vehicle would get HORRIBLE mpg if it went to altitude-no power,and terrible mpg.With EFI/O2 sensors,mass air sensors the mpg actually improves at altitude.
I'm not sure about the prevailing winds-you more or less went East to West,then west to East to El Paso.If the winds were at your back going to CA(good mpg),then they would be in your face going home.
I have been thru El Paso several times(I-10)- about 100 miles E of El Paso, on I-10-there is a big wind farm-the wind blows hard out there!
14-15 mpg is better than average for city driving.17 mpg is about what you would expect at 70 mph-if you are doing 60 mph,then you should get better than 17 mpg. A few years back,the K&N filters were "killing" the mass air sensors because they were being over oiled,and the oil was doing in the sensor.I don't think that an air filter will improve part throttle fuel economy(assuming the old filter wasn't gunked up), but many,many folks disagree with me on that point.
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