2004 Ford Mustang Just runs bad

My daughter got it used and it just runs bad. She has been driving it for 4 years but she says it uses too much gas. I noticed it runs rough. I did have all the belts, spark plugs & wires replaced. Not sure about what makes an older car run bad. I think the transmission make a little too much noise as well but runs with no problems. I know there is a lot here but not sure it there is anything to do at all.