Easy way to get 6000 feet. 1) Max upgrades 2)DON't hold space bar down. 3) As you leave the ramp accelerate to as close to 200 as you can get keeping resistance to about 15% 3)Keep boosting to 200mph untiul your rocket runs out. 4) glide on 0% resistance. Make sure your only just 0%, if yoiu feel unsafe just do 1% 5) when you reach about 10ft from ground pull up to 10% resistance for a bit. Repeat should easily get you 6000feet.
My record is 7300ft Look on Highscores :)
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