I live in the hills of southwestern Pennsylvania. I've had my III for 3 years, and have averaged around 43MPG. I took my kids to a church camp in New York, near lake Erie. While traveling on the rather flat interstates and side roads I was averaging 63+ MPG. That's huge for me. But, as soon as I hit the hills of I-79, I was back down to 55MPG @65-70 mph. About an hour from home, I decided to "go with the flow" of Sunday traffic (closer to 80mph) and the MPG dropped to 46. I've found that using the cruise control on the interstate, in the hills of my area (PA/WV) returns higher MPGs.
On a flat terrain at highway speeds not a ton can be done to increase FE. I use CC while on the interstate a lot too. Even on a mild hilly area methods won't get you a ton higher return than CC. Speed is what kills on highways.
I did 14 hours the day before from Myrtle Beach to Pittsburgh... I was done being locked in a car, and was looking forward to getting out of it...LOL
Getty 43 mpg in the mountainous/hilly part of PA isn't too bad. In PA, driving long uphill road segments -even with a good running speed from the bottom of the hill- makes it difficult to keep the MPGs up. Almost every year I drive through PA to go to MI from MD - it's a real challenge to keep the overall MPGs up in PA. On the way back downhill - gliding the Prius and put the tranny in "B" can store some of that gravitational downhill energy into the HV battery but the battery will only hold so much and once the glide speed goes over 45 mph you'll lose part of that going downhill potential to kinetic energy to aerodynamic drag forces. After a Prius is fully warmed up, the Prius seems to get its best MPG between 25 mph to 35 mph on gently rolling hills with no stop signs-traffic lights.
I laughed when someone scolded me for allowing the HSI gauge into the red. So, not long ago, I turned onto my hill, a 30 MPH speed limit, and drove up it without going into the red. 8 MPH is as fast as I could go... LOL
It's a typical hill around here, nothing special. My friends that bicycle ride up it... I would never think about it. My GPS app says my driveway is at an altitude of 1,124 feet. The firehall at the bottom of the hill is at an altitude of 881 feet, only 786 yards away - less than half of a mile. So, you math guys can tell me how steep a hill is that goes up 243 feet in half a mile. My guess ~10.3% (243/(786*3))*100
