This isn't really possible with the default Grasshopper bike (or at least I was never able to get near the target with it). However, the Pacebreaker bike is fast enough to get the achievement.
If you haven't unlocked it yet, see the text in the spoiler tags below:
Now what you're looking for is a long, straight path that is preferably downhill and lay on the boost (
). If it's too steep, though, your bike will go airborne and you can't boost.
I was able to get around 90+ km/h on many tracks, but I finally popped the achievement on Mountain 3 Trail 2. The trail has a lot of excellent downhill sections that are pretty straight, but in particular Section 4/7 starts with a very long and perfectly straight rock bridge. Line your bike up, lay on the acceleration (
), and you should get 100km/h (watch out for the rock about 3/4 of the way through the bridge). The achievement won't pop until the level ends, annoyingly.
If you're having trouble with the Pacebreaker, the Javelin is even faster, but it is a later unlock.