There are two major shared standards, the American EPA testing cycle and the European NEDC testing cycle. Both are based on essentially taking a car out and driving it in a certain way until the battery is dead, and seeing how far it got.

The EPA range is fairly realistic in my experience, at least with a Tesla. Hills and climate control (AC not so much, but heating is huge) will affect it, but I get fairly close to the official numbers much of the time.

The NEDC test is much more gentle and provides wildly unrealistic numbers. You won't achieve NEDC numbers unless you're doing something like a steady 55MPH on level ground with no climate control.

You can see this in action by going to Tesla's design page and switching the country between the US and the UK. For the US, the Model S 60 is rated at 218 miles and the P100D is 315 miles. In the UK, those numbers are 253 and 381.

BMW says 300km, or 186 miles, for the larger i3 battery under the NEDC:

http://www.bmw.com/com/en/newvehicles/i/i3/2016/showroom/ran...

But the EPA range is more like 125 miles.

Tesla's stated Model 3 range of 215 miles is the EPA rating, and so should be fairly realistic in mild conditions, less so in the winter or with a heavy foot. The BMW number to compare to would be 125 miles.