if you had already played this before they added achievements, and are having trouble getting achievements to show up, try completing a level, then reloading the game. that fixed it for me
6000 feet is quite possible. It is 6000 feet distance, not altitude, in case there is confusion there.
The most popular method seems to be to max out everything, level out to 3% resistance, and use rockets to get your speed over 200. Whenever your speed drops below 175, use rockets to get it back over 200.
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