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I'm just wondering if the humidity can cause lower fuel economy.

It actually tends to improve efficiency because water vapor is less dense than dry air. This translates into measurably less wind resistance -- even at lower speeds!

My history shows a 6% improvement with humid weather, compared to dry.

Florida has the advantage over Arizona. I wonder if the mileage logs could show this? for that matter does the South get better fe than the North? Is summer better than winter? It is for me.

Are you thinking about the water vapor hoax that has been going around? (run your car on water) I fall for a lot of these things but that sounds like "too good to be true" and probalby is. There is a thread here somwhere busting that mith.

Good question. I am interesed in what others have to say about it.

It would be hard to compare FL to AZ drivers due to altitude (98 feet mean vs. 4,100 foot mean) and terrain (flat vs. hilly/mountainous).

Keep in mind that if you're comparing Florida and Arizona this time of year (aside from the valid elevation argument, but that's not what the OP was asking about), the AC likely is working harder when it has to dehumidify. I don't have data to support this, but my gut tells me that that offsets any decreased drag from humid air. I do know in the Prius, the difference in FE between 10% humidity and 90% (with AC off) is less than 1% at 55 MPH. At typical hypermiling speeds (for me anyway) it will be even less because of the overall lower aero drag.

Thanks for all the replies, I was hoping for the opposite answer. I'm still trying to find a reason for my sudden drop in fe since my last fill up so at this point I will have to assume it is ethanol related or bad gas.

Humidity when hot really forces me into A/C, thus less mpg. I aslo am getting a sudden bad drop in mpg. I'm probably gonna blame ethanol gas, because my coworkers have noticed less mileage too with the local area TX gas.