yet I am starting a project where to continously track the speed of a car.
Continuously as in 100% coverage by GPS speed reading?
This makes it sound as if you just need to know when the vehicle starts and stops moving:
For example, if the speed is ahead 10km/h the app will capture the current dateTime and if it is below 10km/h as well.
in which case, if the phone has (3 x) accelerometers, then you maybe you can use those to detect when the phone starts moving, and then when that happens, switch on the GPS, which might take 30 seconds to get an up-to-date fix, but:
presumably you know the location of when you were stopped, and you know that you started moving 30 seconds ago and where you are now, and (average) speed = distance / time
would that be good enough for your purpose?
or for slightly better accurace: you know you started at 0 speed, and in 30 seconds you've travelled x metres and are up to y speed, so you could assume constant or typical acceleration, and work it out based on that, eg say:
from a standing start 30 seconds ago, our first GPS reading is that we've travelled 200m and are now at 60 km/h
a truck accelerates say 80 km/h in 20 seconds = 4 km/h second, thus 15 seconds of acceleration time x average 30 km/h = 125 metres and the other 75 metres... hang on, don't need to do all this, if after 30 seconds we are at 60 km/h then you could interpolate that after 5 seconds we were at your 10 km/h stationary/moving threshold, and presumably
within a couple of seconds is close enough.