Immediately after Luffy enters gear second for the first time, during the fight against Blueno:



Blueno: Whats that mist? Is this his imitation of a steam engine?



Luffy: Im glad i met you all here. Thanks to you, my nakama will be safe for a long time.



Blueno: Isnt it because of us that youre about to lose your nakama?



Luffy: When i fought ao kiji, i thought about it. If strong guys like this are going to appear on our road ahead, ill need to become stronger. Im glad that i met you all here. Because of you, ill be able to protect my nakama for a long time.



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Conversation between Blueno and Luffy points towards soru inspiring his ideas for the gears. If that is false, it would mean that Luffy thought the speed of soru alone would make the difference for saving his nakama, and not the gears.



If Luffy could've used gear second or third you can be damn sure he would've used it in the mansion before Robin jumped out the window.



There is no explicit proof given that it wasnt learnt on the journey to water seven, but there was no hinting to new techniques at all until after the mansion fight.



Seeing the feet tapping for soru which increased blood flow, gave him the idea to do the same using his rubber legs, creating gear second.



No explanation at all was given on how he could have possibly conceived the idea of gear third, and the only info given was Lucci's analysis and Luffy's thoughts during their battle. One can only speculate the idea of pumping blood through his veins for speed led to pumping air into his bones for power



I agree with almost all maaghms' points, except for one: When Luffy was in the mansion and got tekkai'd and wasnt strong enough to damage, i find it extremely unlikely he wouldnt have tried to use gear third if he had any idea how. I agree, its huge size is a bit of a stretch to fit on the train, but its not like his ship is any bigger or more convenient to practice on.



As for the naming system, it should just be ignored since no basis is really given for it anyways.



And as for "but the reason why he didnt use it earlier in water 7 is because i dont think its wise to show your enemy your trumpcard when you know nothing about them. also it took long years for them to master soru, and i think going into gear 2 gave luffys leg muscles the strength to unlock "soru" and it pumped blood around his body faster greatly increasing his metabolic rate."

- Trumpcards are made to be used before you get the snot beaten out of you and your friends stolen, and i dont see how a faster metabolism is gonna win any fights.. unless he starts off really bloated?



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Top half fact



Bottom half opinion