Why is my car so bad on gas?

I have a 1994 Nissan Altima Gxe Sedan. Automatic. The car will get 400kms out of 45Litres or 248 miles to 11.89 gallons. The car has a problem with the transmission: To change gears you have to let off the gas at 3000rpms or so and let it go back down to 2000rpms, then put on the gas and it changes at 2500rpms. That is to get out of first. after that if you let of the gas it changes with ease. When I'm on the hwy at 120km/h or 75mp/h i will be at 3500rpms on cruise control. Would this have something to do with pour gas mileage? If so how would i fix it, and would there be any other contributions towards pour gas mileage?