Well, thanks to the Guardia Civil, we finally have the answers.
In a post on Twitter, police confirmed that on any speed limit up to 100km/hr, you can add 7km/hr without being snapped.
For example if the speed limit is 50km/hr, you can travel 57km/hr, if it’s 80km/hr you can travel 87km/hr etc.
¿Sabes a qué #velocidad real te denunciará un radar?
— Guardia Civil (@guardiacivil) February 9, 2018
On speed limits from 101 km/h upwards the leeway is 7%.
So if the speed limit is 110 km/h, you will be snapped driving at 117.7 km/h.
Police reminded drivers ‘the road is not a racetrack’.