Fuel Economy

Okay, how do you do it? I, too, own a 2010 Prius. I've only driven it a total of 4,900 miles in the nine months since I purchased it new. I live in the middle of the country. I stay in ECO MODE most of the time. I accelerate gently. When the light turns green, the vehicle ahead of me typically leaves me in its dust, and the vehicle behind me is usually impatiently riding my rear bumper. My overall fuel mileage has been 45.5 mpg. On my first road trip with the Prius last July I got 52.1 mpg on a trip of 170 miles, and 52.2 mpg (my highest ever) on the return trip. I was feeling pretty good. On a 400 mile trip in August I got a bit less than 47 mpg. I wasn't feeling so good. On a 1,300 mile trip in September I got a tad under 50 mpg. My most recent highway trip was last November, 650 miles round trip. I averaged just under 41 mpg each way – very disappointing. I routinely set the cruise control to 72 mph on the highway. All of these trips were over relatively flat terrain, with no significant winds. Much of the November trip was through moderate rain. During the winter I've generally averaged less than 40 mpg on short trips (usually around three miles each) around town. I frequently need to move the Prius from the driveway to the street just long enough for my wife to back her car out of the garage and be on her way, then move the Prius back into the driveway – total distance traveled is about fifty feet. I expected that I would be able to accomplish these mini trips on just electric power. Despite the fact that I only relax pressure on the brake pedal and never touch the accelerator, that I am moving at two mph or less, that the traction battery typically indicates that it is charged to 75%, and that the HVAC system is off so that I am not calling for cabin heat, the ICE fires up every time. Why is the ICE starting up under these circumstances? Help!