I've done some long trips with my i3. The key is to turn the engine on when the battery reaches 75%. The battery then absorbed the large power draws when accelerating or going up big hills. The engine does vary in RPM depending on my power draw, and by the end of my trip, the battery is usually not significantly lower, maybe around 60%. I usually turn off the engine when I'm almost at my destination so I can use the remaining battery energy.