In Borderlands, what is the fastest way to get 5 seconds of hangtime in a vehicle?

Solution 1:

Melee off Piss Wash Gully (Earliest opportunity)

If you have a partner Melee your vehicle while you're in it on the Piss Wash ramp, that also unlocks it. (Youtube Video)

Dahl Headlands

I've gotten it in the Dahl Headlands off the beginning Ramp. You've got to be lined up perfectly.

Rust Commons East

There's a bridge near the Middle of Nowhere that (at one point) during the game is lowered for you (I believe it's on the mission to go to Janistown); If you talk to the Claptrap, and then back up the racer, you'll get insane air if you jump the bridge while it's about 50 degrees or better.

General Knoxx's Armory

The highways in the Sunken Sea provide good opportunities. The easiest way to do it is to get in the racer and launch your self off the left end of a broken road (where the curb hits the road). The hitting of the curb makes you go higher and makes it more likely.

Solution 2:

I did it by sliding the runner on its side while it was moving. This gave me the achievement even though it was a glitch. It it is very difficult to do, but you can do it as soon as you get the runner.