I spent several hours trying legit jumps and only unlocked the 25meter and 50meter achievements. 70% on the 75meter was as high as I could get on that hill by the Training Center.
I reached out to a couple players who I saw unlocked this one and got a few replies, all stating it was Pure Luck.
MrWizard1O4 had a capture video in his reply so i tried to duplicate that scenario.
Mack Granite with a MEILLER P330 rear tipper body
Fill with Sand at the Gravel Pit
Head back towards Desert Springs
And go as fast as you can around that circle drive/round a bout and continue
towards Desert Springs and hit the Speed Limit sign and it'll flip you up into the air and all around.
I tried 3 times and got it to fly just far enough to unlock the 75meter(s)