They told me it was the Mass Air Flow sensor. I wish I knew more about cars to tell you the difference lol. That is what they said though. He said that is probably the reason that I am having problems with the bad gas mileage.

 

^ Well I hope for your sake that they are right and it fixes the problem.
The mass air flow sensor is what tells your engine computer how much air is going into the engine. It's part of what helps the computer decide how much fuel to use.
To get a perfect burn (and therefore the most power and best economy) there is a perfect ratio of air to fuel (Air/Fuel Ratio or AFR)
If you're mass air flow sensor isn't doing it's job right, the engine computer isn't getting a proper idea of how much air is coming in so it's not sending the right amount of fuel. It may be running rich (too much fuel) or lean (not enough fuel) but either way, it can make you loose economy.
I'm just a bit surprised the MAF got far enough off to cause your economy to go down that much without throwing a service engine light.

 

I never actually thought about it that way. That is true. They asked if I had any engine lights appearing. I did not. I have never seen one. Let's see what happens. I have a warranty for another week after this so I hope this does the trick

 

So even after the Mass Air Flow sensor was replaced, it looks like that did not improve the gas mileage one bit.

 

Tire pressure didn't help either? I've just recently bought an '06 G35x and get 18-20 (70% hwy) on winter gas (in my experience, down about 2 mpg from "summer" gas) here in Iowa. For winter mileage that's a little lower than I was hoping for but I'm still experimenting with driving style. My last car was an Acura TSX and got 25-27 in the same mixed driving.

I'll need to experiment to see how high I can get it on the highway -- need to stick with 65 mph.

 

Yeah, good luck with that.

I cleaned my K&N filter and wish I would have done it before my last trip. It was pretty dirty, which I'm sure was a factor in my lower-than-expected mileage. Of course, going 75-80 most of the way didn't help either.