Prius veterans remember a fantastic fuel economy marathon some years ago using a G2 Prius. The car was filled to the lips with fuel and driven until empty using local roads on a ~ 30 mile or so circuit. Every couple of hours one of four (or was it five?) drivers would take the wheel. IIRC the car was done at around 1400 miles; the highest MPG leg was about 125 mpg by Wayne Gerdes. That record has since been eclipsed in the US, while similar records have been performed in Japan. So 140 mpg is possible, and it does not mandate downhill driving. It does imply either good wind or slow hypermiling. Certainly a round-trip value is more impressive. Regarding the possible descent: If we start with 1200 kg mass, every 100 meter descent is mgh = about 1,200,000 joules or 1/3rd kwh of potential energy. If the car ran at best thermal efficiency of 38%, a 100 meter drop saves 0.876 kwh of fuel, or about 0.026 of a gallon. These drivers used about 0.6 gallons, so a 100 meter descent would improve their fuel fuel economy ~ 4.5%.