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MPG's increase with temperature, humidity, and altitude. As mentioned above, air density decreases with increasing temperature. Since the water molecule (molecular weight 18) is lighter than molecules of nitrogen (28), oxygen (32), or Carbon dioxide (44), as the percentage of water vapor increases in the atmosphere, its density decreases. Also water vapor displaces oxygen that your engine would otherwise be burning, reducing its power output and forcing you to open up the throttle (for reduced pumping losses) more to get the same amount of power output. You also get reduced pumping losses as the air density decreases with increasing temperature and/or altitude.
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