I found a pretty sure fire way to get this is by driving off the cliff on the west side of the map. It'll warn you about driving off the map and will reveal the location "The Big Nothing". Keep driving off the cliff and should be easily 4 seconds. Me and two friends have got it this way.
Drive to where the crosshair is....
Edit: Grimace221 kindly added.... For those wondering how good the car has to be, I had the best V6, the tier 4 suspension, tier 5 exhaust, and boosted right as I hit the ledge. This can probably be done with a lesser upgraded magnum opus. Also worth noting if you need more air time, try stripping down armor, and spikes, etc to lower weight.