Nothing has as much effect on poor economy as a dirty air filter.

How exactly are you.calculating mpg, taking miles traveled from the odometer and dividing by gallons of fuel put in the tank during a fill up yes? Just making sure you aren't using a software that calculates it automatically like the OEM nav unit does.

Ever check your tire pressure?
Ever check for junk in the trunk?
Do you warm up your vehicle for long periods?
How do you calculate your MPG?

When I had my 06 G35 6speed sedan I did one month tests of 87,91,and 93 octane when commuting 2k/miles per month
I got better MPG using regular 87 octane and have used it ever since.
I recently returned from a round trip from NJ to Florida in my Q70 and I get double MPG from what you are getting using regular 87 octane.
From my fill up from NJ to Va. 293 miles and 11.729 gallons to fill up =24.98MPG
From Va to SC 297 miles and 11.671 gallons to fill up =25.45 MPG
From Exit 1 in Georgia to Lake Worth Florida 327 miles and 12.718 gallons to fill up =25.71 MPG
I use Loves, Pilot, and WaWa regular gasoline and no cruise control
Tires inflated to 35lbs when cold
Either you have something wrong with your vehicle or its your driving style?

Telcoman

I'd check your fuel trims to see how the engine is performing. I always got crappy mileage using 91 octane and have used 87 for years without any issues. When I did get poor MPG it was from a vacuum leak at idle and after I fixed that it bumped up to the 17-19 range now.

You would need a scan tool then hook it up to your OBD2 port. You can get an ELM bluetooth and Torque for Android to start off with for basics. You'd look at your short-term/long-term fuel trims when the car is running at idle. The long term are the baseline for how much fuel to add/remove. The short-term are the instant readings in real-time and both of them should be close to 0 in a perfect world and around +-25 the ECU will set a code. So, if you're ECU is adding/removing fuel at say 20% it could not even set a code. Then you would want to rev the engine to like 2K RPM and see what the fuel trims do. If they get close to zero or have better readings that that's a sign of a vacuum leak. I used a vacuum gauge to find my leak at idle my engine would rev up in a pattern and then the gauge would drop from like 22 down to 18-19 and back to 22. You can also use unlit propane, brake clean, around the intake area to see the engine acts different.

If your car is tuned though, I would start there first to make sure it's configured correctly.

Any time I have issues with SLICK I go back to my tuner, he plugs in his lap top and fires her up! That way there's zero guessing, unlike going to the dealership! I'm not really concerned about gas mileage, only the cost here in Ca.! What I did find, using my cruise control gets me 300 miles per tank rather than driving around town which cost me 30% less...use it when you can! Unless you're using a K&N filter replace your OE drop in every 10K miles, more frequently if you live in a dusty state like Arizona or Texas
Good Luck & enjoy the ride...Gary

Yeah when you said you cleaned your filter, do you mean that you used the recommended solvent and oil to clean a filter that's DESIGNED to be cleaned like a K&N?

Or did you use an air compressor to blow out a paper filter that's NOT designed to be cleaned because that won't work.