In order to unlock Speed race IV and get the 1200 mph achievement, maximize your Engine and Boosters Power. I got 293 engine and 244.8 Boosters. Then just do a vertical flight at full speed and pray that you don't hit something too early.
I wish there was a proper ending and not just going back to mission screen after you beat the last mission. Also the game becomes cake once you unlock the M26 Pershing. Just focus on upgrading damage, accuracy, range and maybe reload (you only get 10 upgrade points) and there'll be hardly anything that can touch you.
It's possible to get stuck if you dig deep enough so that you can't jump out anymore and don't have any teleporters left. Instead of waiting for your fuel to run out there should be a suicide button.
Lv 10 has 125 enemies and Lv 14 has 116 enemies. The numbers are just displayed incorrectly. You will earn the respective stars if you shoot down 125 and 116 planes. The planes from the carrier at the end of Lv 14 do NOT count towards your total.
I think the game can be legitly beaten in 2 days but on day 1 you're simply not fast enough. You absolutely need 2 speed upgrades. And then a lot of patience.
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You don't have an inventory until you beat the dungeon that you fell into. And you can't fall into that hole until you're ready.