Sorry for party pooping the Tesla hate train, but the milage estimate when navigation is of is directly correlated to battery state of charge. Its basicly just SoC x factor. Its not dynamic, as in a Kia or BMW. The factor is calculated from officoal EPA range test. Should it be dynamic? Maybe, but you get a true estimate when you navigate to a destination anyway. This is probably done so they could market the car with certain range, same as many other manufacturers.