How is speed efficiency/temperature efficiency calculated in detail?

Does anyone know how precisely the speed efficiency or temperature efficiency is calculated? First of all, the car sends data every 30 seconds and also has precise odometer information and battery range estimations, but are the efficiency numbers calculated on a per trip basis or every 30 seconds? 

Per trip example: find out the start and end odometer, start and end battery range, and average speed, to find the efficiency vs. speed graph. Cons: average speed might not paint a good picture if you spent most of your trip at 10mph and 50mph and thinking that the average 30mph is where all the data is referring to.

Every 30 seconds example: find the odometer difference, battery range difference, as well as speed difference after 30 seconds calculate efficiency from there. Cons: sometimes the odometer or battery range doesn't move in 30 seconds, leading to some divide by 0 errors.

Which one is the likely one to be used in TeslaFi? Or is it a third method that I didn't think of?