It's the opposite for me^ over the summer, if I put the ac on..forget about it, i got like 8 mpg lol
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I also heard..and experienced that when I put my ac on a lower number such as 66 opposed to 74 I got a worse mpg too. My dad told me to keep it around 70 because when you put it on low your ac has to work harder to make colder air.
I can't believe this claim - the AC does not impact efficiency on the FX35, or MOST modern cars. Lowering the temperature impacts mpgs? I laughed when I read that. Not a chance. You have something else wrong with your car if you are getting 8 mpg. Your AC compressor is belt driven off the motor even when you have the AC off. There is a minor change in load on the pulley but not enough to notice on the trip computer.
I'm an engineer and a big believer in a controlled test of this kind of thing (I'm a bigger believer in my own experience). In order to take this seriously you would need to drive the same way for a couple weeks in a row with the ONLY change being your AC. Temperature outside needs to be the same, similar times of day, same routes, same traffic, etc. I've done things like this several times because I have worked at the same place for 12 years and can really feel anything off in my car because I know the route so well. One of these controlled tests I did was regular vs premium fuel. Premium fuel won - it is CHEAPER than regular because the FX gets better mileage on Premium, enough to offset the cost of the higher grade fuel.
To the OP - I would check your spark plugs because the valve cover seals are known to begin leaking around 80,000 miles - allowing oil to collect in the spark plug well. Even if there isn't oil, you can check the electrode to see if there is carbon buildup or any evidence of damage as compared to a new plug. That can definitely impact mileage.
If you are still willing to investigate, on your next oil change, send a sample to Blackstone labs. For around $30, they will tell you what is going on in your oil - if there are high levels of anything you should be worried about. Check their website. You could try running a high detergent oil for an oil change cycle or two - Shell Rotella T makes motors look like new on the inside.
