Andy, if you have those STEEP of hills, I learned a new trick that could help you out a lot.

On the downgrade, shift to L to get the engine to run-up to 3500-4000 RPM.
Wait at least 5 seconds. This will cut all fuel to the engine, even though it still spins, it will spin by electricity only. You can even keep the A/C on during those hot days, and use no fuel. If you want the slowing that L creates, leave it in L. If you DON'T want that much slowing, shift back to D after a few seconds of high RPM. The RPM will drop. The regen will decrese, and your speed will increase. Still, no fuel will be burned until the next time you press the gas pedal, or, the engine gets cold. ( probably not a problem if your car is at full temperature before you do the downgrade )

Good luck, and report back if, and how much this helps your MPG. You can use this trick at any speed. I've used zero fuel at 70 miles per hour on Colorado highways with steep downgrades. This is just like EV at 70 MPH!
-John