It's a cool idea that I've been toying with as well, but when I mentioned it to friends here in TX they all responded the same way: be prepared for some nasty action by other drivers. Even without such display, there are some drivers (mostly of giant SUVs and jacked up trucks) that have to show to you they don't care about mileage by pulling to within inches of your rear, or by cutting you off as close as they can as they pass (the same happened in my former MINI Cooper, another car that seemed to irritate a lot of SUV and truck drivers here in TX).