Idk, but you should have done some research before buying your car. The v6 mustangs have a bad reliability record. The v8's on the other hand, blow 99% of cars on the road out of the water and according to JD power and consumer reports have an excellent reliability and repair record. Also, if you are being honest and you have never gotten more then 20mpg on highway driving, you either need to let a mechanic know so your car can be tested or you have to go to a driving class. If you go to car and driver and look at there back issues, I believe its either April of 1999 or May of 1999 they did an article on a 1998 v6 mustang
and got an average mpg of 23.8. They got 26.7 on the highway and 20.8 in the city. So unless your cars broken, its because of the way you drive that you are not getting the listed MPG.