Fuel consumption

As you can see by my profile I have an '03 Mitsubishi Lancer Evo VIII, and I'm curious to know if I have a fuel consumption issue, from empty to full it takes 30 dollars of premium grade gas at 2.93 a gallon due to the turbo I have , I was told it is about a 243 horsepower engine, it is a 5 speed of course

but the thing is, I probably run the car about 3-4 hours a day, one hour of which is on the highway going 30 miles.

After about 2 days I'm down about a quarter to half a tank of gas, and I am curious to know if this is normal or not, I was told at the dealer ship that it gets about 23 Highway miles to the gallon, and I've alreadty brought the vehicle back to the dealership a few times with other abnormalities, such as brake squeal, dead panel bulbs and a loose heat shield.

Any help or troubleshooting tips would be very much appreciated.
The vehicle has about 86,000 miles on it, it is a USED vehicle

The 03 EVO 8's gas mileage is not good. It's tuned very rich from the factory. I'm averaging 18 MPG.

Man, I put 20 bucks in which went from just under quarter of a tank to just under 3 quarters of a tank, I went about 90-100 miles and i'm running on empty, 20 bucks to get that much :/