Before your first usage of the app it is VERY important that you go into Settings and set the proper settings for your car. You need to set what unit to use (metric (km) or imperial (miles)) and what your car's typical calculation value is.
So when supercharging, you can enter the distance to the next supercharger and then your battery charge (typical range) and find out what consumption you can average on the way to your destination. That way it is very easy to see how much typical range you need!
Note: This is not an official app by Tesla Motors, the app is not affiliated with Tesla Motors in any way. Tesla holds the copyright to everything related to the Model S and the superchargers.
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