I dunno either. How long are your average trips?

 

Its a combined 50/50 driving. I don't let the car idle to warm. Some trips are long and some short, but average is about an hour.

 

Actually my cumulative gas mileage (miles driven since got the car/total gas purchased) has gone up a little bit over the last couple years. To over 21mpg overall for my 04 sedan.

There have been a couple variations, either of which are probably contributing.
I try to buy 100% gas when possible. 3-5% improvement in energy.
My work is not 8 1/2 miles away instead of 4 1/2. And I get on the interstate for 1/2 of it now. So the car has a better chance to warm up.

 

fluids are a big factor in your millage.

change engine oil to a better one (Castrol German Syntec 0W30 is one of the best)
change diff oil to a good synthetic Amsoil fluid.

 

just drive it!

 

Calculate your MPG using JOKER's method. My 05 Sedan 6MT gets about 19MPG. I get about 300-320 miles and I usually pour 16-17 gallons every time.
I pour 76 gas most of the time because it's close to my house. One time I had Chevron gas in the tank and I got about 21MPG, not sure if it's the gas or maybe I was just easier on the pedal.
My car has about 106k.
I changed my spark plugs to NGK Laser at 102k.

 

Is your check engine light on? maybe you have a problem with your evap system.

 

Driving style.

 

Honestly yeah do Jokers method. I did it your way too when I first got my G and I got around 15 mpg.

But I filled it up, drove til the light came on, and filled up.
(Divide gallons filled up) / ( by miles driven) = your MPG

I got 21 MPG this way lol. And using this method I found out my car fills up to 14.5 Gallons. So when my gas light comes on I have around 5.5 Gallons of fuel left. our Gs have 20 gal tanks.

Also it could depend on where you are. We get 91 here in CA so if you got 93 you might even get better mileage.

 

You might want to invert that.

 

check your pcv valve, i was getting terrible terrible milage, replaced it and had a huge jump in my mpg

 

possibly need an oil change...

 

I live in Chicago and drive for 1-10 miles at a time in stop and go... 12 MPG seems pretty common for our cars on that type of driving...