Anyone have thoughts on how long the battery pak will last in these? I bought my wife's 2014 hybrid Z recently when she bought a new Maverick hybrid. With gas prices the way they are, $6.39 now for reg here, I wanted the nearly 40mpg of the Z. But I'm 'only' getting 37.2 I'm thinking maybe the batteries are dying and the gas engine has to run more. Anyone have knowledge or thoughts on this?