Altitude DOES have an effect, and it actually should improve your gas mileage. The air is less dense so the PCM is going to meter in less fuel to keep the A/F ratio correct. Of course this also give you less power, but once you reach cruise speed, you don't really need much power.
A big consideration, and one that is probably effecting you is the altitude change. Going uphill takes a lot of energy, more than driving on flatland, and you don't get it all back when you come back downhill. That would imply 100% efficiency. So, climbing and descending hurt you more than just cruising on level ground.
Winding roads will keep you in lower gears too, which also hurts mileage. Mountain roads tend to be "twisty".
- Jack
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