I have a 2000 Insight with about 58,000 miles. I live in upstate New York. I topped out at a lifetime mpg of 63.4, but have gradually seen the lifetime number dwindle, now to 62.5. Since this is my fourth winter, I'm aware of the plunging winter mileage phenomenon. Though nothing sudden has happened, clearly I am getting worse mileage than I used to, something I started noticing last summer and fall. All conditions - tire brand, pressure, oil, driving routes and patterns are the same. Could this be a sign of decreasing efficiency in the battery pack?