The trucks gain/gather 'speed' as they drive down the hill. = The trucks pick up 'speed' as they drive down the hill. [=the trucks go faster as they drive down the hill]
He types at a 'speed' of about 50 words per minute. = His typing 'speed' is about 50 words per minute.
Let me 'bring you up to speed' on our plans. [=let me tell you about the changes in our plans]
(..) Our list of allies grows thin. By foul craft,Saruman has crossed Orcs with Goblin-men. He's breeding an army in the caverns of Isengard. An army that can move in sunlight and cover great distance at speed. Saruman is coming for the Ring. This evil cannot be concealed by the power of the Elves. (..)2001 The Lord of the Rings: The Fellowship of the Ring