Well per his location he lives in Texas. Thus he is running his A/C, most likely lives in an area with fairly high average wind speeds, and is not living at elevation so air densities would be higher (my car gets much better mpg in Colorado or Wyoming than it does in KC for example). Moreover, if he lives around Houston, then all that smog and filth in the air there has got to add additional drag to his vehicles.
That all said, warmer climates typically give you better mpg in winter than colder climates since when its really cold out your vehicle idles at a high rpm for a long time until it warms.
The fuel mileage difference you see is because the fuel/air ratio is set to around 14-14.5/1, optimum is between 12-12.5/1. When you go up in altitude, the air is thiner, so the computer tries to adjust, but in doing so, you actually get a more optimum ratio. The manufactures set them rich like that so that there is a better ratio at the exhaust under the old standards. Further, timing is not set to optimal either. You see, if you set timing and richness to optimal, you get better fuel mileage but you also get a higher CO2 reading, since the better the burn, the greater amount of CO2 and water will be produced during the emissions tests. Higher CO2 is actually an indication of a more efficient burn. High octane fuels, which allowed compression ratios of 11-13/1 produced more CO2 because it was a more efficient burn. A low compression burn, such as in the catalytic converter produces much less CO2.
And no, I don't live in a different universe where physics is different. However, when able, I do adjust fuel/air ratio and timing. Also, I add multi-spark ignition for a more complete burn below 3,000 rpm as well as a higher voltage coil system and lower resistance/larger spark plug wires which gives a much hotter spark.
You apparently think that the EPA allows auto makers to produce the highest efficiency engines. However, short of direct injection, Pre-1996 automobiles with a few adjustments can get significantly better fuel mileage than todays cars.
Other things can affect fuel mileage also. Open up your exhaust system more, headers, high-flow catalytic converter and high-flow muffler with large pipes will give you better fuel mileage because it allows the engine to build better torque on the low end, where your car most often runs.
Colder air is denser air, same as lower altitudes. Your Mass Air Flow sensor, and on GMs at least, coupled with the Manifold Absolute Pressure sensor determine the air volume/mass and the computer adjust your fuel injectors for it.
By adding a gear splitter or under/overdrive system (not available on front wheel drives) you can also change the gear ratio to lower the rpm range for on the highway. Of course, your engine actually has to produce enough torque to run it.
And yes, I run my a/c. Contrary to popular myth, you actually get better fuel mileage with the a/c on during the summer because opening the windows causes a disturbance in the air flow causing more wind resistance and drag.
No, I don't have to deal with the smog of a city. I live in the country. Lots of trees around to soak up all that bad **** your car puts out. Want cleaner air, plant more plants.
If the EPA thinks my car is getting 24 mpg and it's getting 30, that is a huge difference in grams/mile calculations. Luckily, we don't have to do EPA checks other than visual inspections in my county.
