fuel mileage cut in half. my mpg use to be 23 down to 14

bought this car with 69,000 miles on it and a couple thousand miles later the average mpg went down to 14 mpg. I have to fuel up every three days. Most of the time I witness the fuel gauge moving as i drive every ten miles down the freeway going about 70mph. I did a tune up of a new air filter, new spark plugs, coils, and intake gasket and still got the same results. sometimes i do notice when i first start the car it takes a while to crank and then starts but the results look good on the computer side from O2 sensors. 1 person did say it maybe a fuel delivery problem. maybe a fuel pump