Thats a fictitious test number from a test to allow you to compare relative ranges of EV's. A bit like MPG - you are not going to achieve in most real world driving.
The range in the car is based upon 310 miles (for a LR AWD) when new. That is based upon an efficiency of around 225W/mile. If you drive under that, you theoretically will get better than 310 miles, over that, less. Lots of things work against that, right foot, speed, weight, cabin heat/AC, temperature etc etc etc.
I say theoretically, because you do not drive an EV 0 - 100% battery. Maybe 20-80/90%. Everyone had their own views on where the sweet spot is for battery, but the only thing most agree on is to never leave the car close to the 0% or 100% mark for any period of time. If you charge to 100%, you drive pretty soon after - its called trip for a reason. You go below 10/20%, you put it on charge as soon as you can.
The other theoretical bit is that the batter will lose range slightly. So the 310 may become 295/300/305 etc - not a fixed range, can go up as well as down. But it will always be 310 when full. Much like a fuel gauge will show 100% when full. On that subject, many people just use % for battery, much like a traditional fuel gauge.
I wish Tesla wouldn't use 348 mile range, but thats what they have to put. It just sets expectations at a pretty much unachievable range. Just like the 310 will likely be come cooler days or faster driving.
An EV is not a traditional car that you may have been use to for many years. It requires a slight change in thinking. Its more like being a student again and borrowing parents car and topping it up with £5 just to cover what fuel has been used. You charge more for immediate needs rather than filling up when it gets near empty.
