Keep the air pressure up in the tires, lower or remove the tailgate on a truck if the bed is uncovered, drive slower, use a lighter foot pulling out, new spark plugs and air filter if needed. I installed a K&N air filter in my '98 Dodge Dakota and gained 1/2 MPG. When you are able to, roll the windows down and let the vehicle interior cool down some before driving. Your A/C will have to work less to keep you comfortable. When winter comes start your engine and let it run for a few minutes and then start driving instead of letting the vehicle interior get warm. Set the temp control in the mid position and the fan on low. Your engine will reach operating temp faster. Your heater core is a little radiator so the fan on high and the temp control to set on max heat robs heat from the cooling system during warm up.
Each to their own on some supposed improvements. I put a Flowmaster muffler on my Dodge. I got a little more power but in the winter I lost gas mileage.
