[Lazarus]

Welcome to the site enjoy the journey.

Since you have an automatic it is critical that you learn your shift points. Once the car shifts you usually have about 5 mph before it will downshift. The best mileage will be in OD at the lowest possible rpm without down shifting. The transmission control unit will learn your new driving style and the shift will come at lower rpms now that you are not WFO from start.

Learn to read the terrain. Use the slight down hills to your advantage. As far as the hills go. Use a bleed technique. Keep the speed constant until you get to the beginning of the hill and then keep the same throttle position and let the speed bleed until it get just above the point of downshifting and then keep the MPH there so it won't down shift.

To get good at the game takes a while. There really is no silver bullet but by building on each aspect of the drive with technique alone you can get some good numbers. Doing easy mods to the car is icing on the cake. If you're doing 25 MPG now, with the way you describe your style, if you look at the tips and techniques and control the foot you should easily get the 10 MPG you're looking for especially when the weather warms up a bit.

Good luck and if you can afford to get instrumentation it will easily pay for it self.