15 seems pretty low, but this is really dependant on driving style. The best ive done with my box - its a tip. . . was 28. 100% pure highway, cool day. The worst I've ever done was 24.3 - but that was still rural roads with a little bit of stop and go. I'd imagine in VA traffic I would get 18-20.
I too had had mileage drop in the winter on my other car - a civic. I've gone from 34-35 mpg in summer/fall driving to 24-25 in the winter. But the 24-25mpg on my civic was heavily in suburbs + lots of AC for defrost.
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