Couple things that will affect your gas mileage. Tires - Affect mileage to a degree, tires with higher rolling resistance and larger footprints will lower your mileage.
How you drive - City driving takes your mileage down quite a bit compared to just cruising on the freeway. Driving in town on the gas and brakes alot also lowers your mileage. When I was driving 2 miles back and forth from my old house to my new house my mileage for that tank of gas was 22mpg.
Right now I've got a 95 GSR which I drive very hard on the freeway all the time, and reasonably hard in the city. By hard I mean the 2ndary runners are open in 5th gear on the highway, and in the city I typically drive 20mph over the speed limit, never getting into the higher RPM's >6k or so. This type of driving usually nets me 25mpg for each tank. The car is tuned properly and the timing and all that crap are set to stock.
Best mileage I've ever taken down was all highway driving to LA on hwy 101 at 120mph. Pulled in 29.8 mpg. I've personally never seen my car get better than 27mpg in the normal day to day driving unless its pure freeway.
My advice is if your counting your pennies just to be able to afford to drive an integra then you need to either make more money or get a cheaper car because you are definalty living pay check to pay check and that is bs. If anything comes up then you won't be able to afford it and lose the car anyway. Gas really isn't that much money (150$ a month for me) so if your worried about that I wonder if you can get the car you want you know what i mean?