MinimalistG 25

Use one of Humba's chopper/copter blueprints to fly all the way to the top of the Nutty Acres area. Once up there, pause and change your vehicle to a custom one. All I personally needed to achieve this was a seat attached to a sinker (found under the fly/float menu. It looks like an anvil to me). With a vehicle using just those two pieces, I was able to hit the necessary speed before hitting the ground. This should also pop Whoa Nelly! if you don't have that already.

As a side note, I found the description of this achievement and the two related ones frustrating. I wanted to know what this "set speed" was. As far as I can tell, this "set speed" is the speed at which you max out your speedometer in the bottom right of your screen. So if you fill up the whole circle, you've got your "set speed."