Ok, I didn't know what I was doing. I just bought my new 2010 Prius the day before I went on a 300 mile trip out of state. Dealer showed me the eco mode, which sounded like a good idea to me. Drove on the expressway, about 70 mph, decided lots of power was not necessary and I wanted the best mpg so I put the car in Eco mode. Drove great for about 250 miles, then all of a sudden, I began to lose power. Fortunately, I was nearing a rest park. Pulled in, was there long enought to take care a Rest Park business, started back up and car ran fine. BUT I did not use Eco mode. Question: When should I use this mode and when should I not?