Hi Yarisman, welcome to the site! Most people on the site try and keep their RPMs below 2500 at all times, you will definitely see a big gain if you do that. Most of those small cars are geared short for city driving, so they don't fair as well at highway speeds. Try keeping under 60 or even 55 mph on the interstate for a tank, you will see that it helps quite a bit. Check out Wayne's article Beating the EPA
for a lot of ways to improve your mileage. You can start by making sure your tires are pumped up to sidewall pressure.