Please remember i drive a 98-2000 LS which is a very different car than previous or later years. Its engine and transmission are made perfect for highway cruising speeds and do not use gas like others.
I use ultra premium synthetic oils, new plugs clean air filter , cold air intake that has air being forced into the intake making the car run lean and cooler. Also i have almost perfect alignment and i keep my tire pressure almost to the max to get the best mileage on the highway.