If your engine misfires, hesitates, stalls, gets poor mileage, is hard to start or has failed an emissions test, it clearly needs something, though a tune-up in the traditional sense might not be the cure.
If you tell a repair shop you need a tune-up, the mechanic should ask why you feel you need one before recommending any service. Just like a doctor should ask what symptoms you’re experiencing, a mechanic should seek to diagnose the problem. And just as a doctor may recommend some tests, a mechanic may do the same.
You can speed the process by being ready to describe what happens and when (such as whether your car hesitates when the engine is cold or when passing at highway speeds), any sounds you hear and what you feel when your car’s “illness” shows up.
One caution about lower fuel economy: You should expect it to go down at least a little during the cold months, and maybe a lot. Colder temperatures make your engine and charging system work harder. In addition, winter gasoline blends have slightly less energy content than summer blends, so they don’t deliver as many miles per gallon. A tune-up won’t make Old Man Winter, or his effects, go away.