Everybody seems to be getting this achievement with the De'Leo Stiletto which I had not come across yet at the time of this writing. I got this achievement very easily with a car that is always available--the Samson Opus. So, I thought I'd show where to get the car and how to get the achievement without using any ramps.
You can get this achievement very close to the beginning of the game. The current mission is
A Taste of the Action where we learn how to set routes. Instead of going to Sammy's Bar in New Bordeaux as the game would have you go, we set a route to a garage in Tickfaw Harbor to pick up a Samson Opus. The Samson Opus is always available in the garage shown in the video. No updates or modifications are required. The car goes up an incline after a bend in the top part of the route. If the car is going at a high rate of speed, it will automatically go airborne. Even if you crash at the end, the car will not flip so you will get credit for landing on the wheels. The advantage of this route is you do not have to use a tricky ramp:
We need not have driven that long a route to get this achievement. We didn't need a very long approach for the jump. So, if you're working towards this achievement by itself, you can start your approach just before the bend in the road. However, by taking the long approach we were able to keep our speed above 120 mph for 20 seconds to get this achievement at the same time: