Many speed stunts are elevated, and I've been using helecopters for it. For the really fast land ones, I recommend Verdeleon Eco. I forget where I unlocked it, but to get it, you have to park one directly under the ring, then use the air lifter. Set it for high under the tap column (or hold, just don't use automatic or it won't work).
Now attach a few air lifters to the car. Get in and press (or hold, depending on your preference)
to unlock the car.
For sea ones, I recommend Mugello Pescespada SX. That can be unlocked at Lago Hatun, on the border of Gran Batalla and Frontera. The ring is under a bridge and the speed boat you're looking for should be nearby.