Note 1: you don't need to upgrade accel and ramp height to max to do the 6000. Just upgrade to full fuel and full air resistance. Then maintain 150-175 knots at 3% until the fuel is gone. Glide. Done. Note 2: its cheaper to buy more than one upgrade at once. Buy at least two upgrades in a row except for day 1. Better: three or four. Note 3: aim for high achievements early. On most ramps, you can do the speed and range achievement later in one flight after you got a better glider. It's a bit tricky to find the right timing for the right achievements, but with this rules it's no problem to get 6000 in 17 days. On a lucky day, I got 15.
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