We were averaging 5.0 l/100 km (47 mpg) on our 2005 Prius that has done 175,000 km. Suddenly the mileage increased to 4.2 l/100 km (56 mpg)! This only lasted a couple of days and routine drives of 15 km - 20 km. (9 - 12 miles) both by me and my wife. It occurred directly after putting half a tank of gasoline in the car when it was under a 1/4 tank. it then went back to 5.0 l/100 km. Does anybody have an explanation or similar experience? We bought this vehicle used just a few weeks ago and we, of course, just love it! It drives great and is so spacious and quiet inside! The previous owner said that he was averaging 4.8 - 4.9 l/100 km and the best he ever did was 4.6 l/100 km on a long distance trip. He lived in the city, so that was mostly city driving, we live in the countryside where our driving is more mixed.
My Gen 2' fuel economy was very sensitive to outside air temperature. About 30 degrees C should be ideal.
I would track full tank consumption and average over several tanks. Like Jimbo said, the Prius is sensitive to temperature (it determines how quickly the engine can shut off). Your countryside driving is more applicable to dense urban driving so you should be able to achieve lower consumption than the previous owner. Pulse & Glide techniques can help too (assuming there are passing lanes for traffic or traffic is light enough that you can do P&G).
