I drove a rented Model S from teslastation.de on the German Autobahn. I found the range to be very close to proportional to the speed, from around 25 mph and up. In other words, at 120 mph you are getting about half the range of 60 mph.
As km/h is used in Europe (multiply mph by 1.609 to get km/h), I found a good rule of thumb:
As "typical range" is pretty accurate at 100 km/h (62 mph), you can add the "speed as percent" to the range required. (adjust for hills, A/C and aggressive driving - brake use)
Example:
You need to drive 150 km, at a speed of 130 km/h. 130% of 150 is 195 (150 x 1.3), in other words, you need
195 km of typical range to be able to go 150 km at 130 km/h.
The same can be done for mph, you just need a little more mental (or calculator) math.
Say 60 mph range is close to "typical range", and you want to do 120 mph. In this case you have to double the required distance, to get the "typical miles" equivalent. To go 100 miles at 120 mph, you need to start off with at least 200 miles of typical range.
In this case, km/h happens to be a bit easier to do mental math on, while mph is still doable. Say you want to go 80 mph, well just multiply your require range by 80/60 (which is the same as 4/3 in this case, adding 33% to the required typical range required). Or you can flip the fraction, calculating how far you can go at a certain speed: If you are doing 80 mph, you can go about two thirds of your typical range.
Charts for mph:
Range for km/h:
As you can see in these charts, range is both
linear and proportional, from around 25 mph / 40 km/h - and up (
almost linear/proportional). At lower speeds, overhead power usages starts to impair the range down towards 0 speed where range goes to 0 as well (you are going no-where, yet you are using electricity just to power the car).
You can also just drive for a few minutes at whatever speed you want to measure it for, then use the Energy screen and average setting.
