Mafia II

Simple! Pick any car, doesn't matter. Travel to the location marked in yellow. There will be a very sharp incline in the road. Hit this going about 25-30mph and you'll have the achievement.

I did this driving through the tunnel closest to the top left hand section of the map.
I used a base model Jefferson Provincial entering the tunnel from the main section of the city.
Gain enough speed driving through the tunnel and there will be a ramp leading up to the exit of the tunnel.
Maintain speed, jump the ramp, and you should definitely be in the air for 20 meters or more.
After landing it right your achievement should pop!

There isn't much traffic so you should be ok to dodge any vehicles in your path.

The first three guides are good and sure fire ways to get this chevo, but I stumbled on a way that you can get this easy and early in the game. Travel to the "Brooklyn Bridge." I used the jeep and climbed up the stairs on to the pedestrian walkway. I doesn't matter which side of the bridge you start on, but it is important to be driving east from the eastern-most bridge support tower. As you drive you will be taveling over a slight decline in the walkway and when you launch over the stairs, the achievmment will unlock. As soon as you drive a car you can get it.

I stumbled on this when I had just finished selling all the gas stamps and was travelling back to Joe's. Just though I would mess around GTA 4 style and was very surprised when the chevo popped.

Good Luck.

Solution One: The easiest way I found to do this achievement does not require any special or even fast car or that you need to be in any specific chapter...

All you need to do is go to the over river bridge in the middle left of the map and then drive up on the middle walk way section and make your way past both of the bridge supports.

When you just pass the second one accelerate to top speed and go off the stairs at the other end and you will easily get 20 meters of air as long as your not in a bus or truck of some sort :)

Solution Two: Another easy way to do this achievement with any vehicle what so ever and at any point in the game is as follows...

Make your way onto the interstate going south bound from the top of the map (Just south of the tunnel that comes from Bruski then you will notice on the right hand side there is a hand rail protecting you from a drop. Pick up some speed then drive right through the hand rail and you will travel far more then 20 meters no matter what vehicle you are in. :)

I just got this one a few seconds ago on Chapter 5: The Buzzsaw, and it was super easy to get.

At the start of the mission, Joe will tell you to go to see an arms dealer named, Harry, who lives in Kingston, to get an MG42. Just follow the GPS to Harry's. Along the way, you will come pass through a long tunnel, that curves about halfway through. After the curve in the tunnel, step on the gas and drive as fast as you can. Upon exiting the tunnel, there will be two hills: a small one and a large one. Hit them at top speed and fly through the air like an eagle, and you will be 10 Gamerpoints and 11 TA points richer.

I accomplished this in the Jefferson Provincial that I stole in Chapter 2, and Joe paid to have tuned.

During Chapter 8, once you have cleared all the thugs at the Old Foundry. Fly like a bird, far far away.

Thank to RoosterTeeth for video

i got this achiement chapter 2 doing my first job for Mike Bruski there was a road right before you get to Mikes junkyard with a bit of jump to it speed up and go for air i got this no problem first bit of playing the game