Rough Idle and Poor Gas Mileage

I have a 1998 mercury tracer. The car runs fine, but there are some minor issues that concern me. The car has a rough idle when stopped, and it gets slightly worse when the cooling fan kicks in (though it isn't really that much of a differece), additionally my gas mileage has been around 25mpg (my car should be getting somewhere in the 30's with the amount of highway driving I do). I've replaced the timing belt and that slightly helped, but I'm still lost as to what could be causing this. I am about due for a tune up, and my mechanic suggested that I get that and a transmission service done because my car was a dealer rental and had regular maintenance done. I'm just wondering if these will help with the problem and if a tune up is really what I need?