operating correctly to maintain correct coolant temps you will have reduced fuel mileage especially if the temps are too cold. I had this issue with my NA Z and checked and then replaced the thermo and the mileage improved to 20 in town 24 highway at 60mph, pretty much what it was when new. Also important to have a clean cooling system and a good radiator.
What fuel are you using? Is it ethanol based? When were plugs replaced? What is timing set at?