You can also write an opinion about the design
The design is fine. Visual antonomasia and metonymy used in a relatively creative way; it works as it is. Not the best design from Oda, but certainly not among the bad ones.
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But he is not, it is Oda's words.
You confuse Luffy with the previous PK.
He is not that guy, pal. Trust me.
Where are those Oda's words explaining Luffy represents fame, Zoro power and Nami wealth? And certainly I'm confusing Luffy with the previous Pirate King; if anything, he will surpass him in fame, wealth... and power.
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I didn’t say all cars have them, but the fact that some of them do might’ve been enough for Oda to come up with the concept of Gears. Gears do different things, just as car gears do.
What else would’ve Oda taken inspiration from?
You pretty much did say that all cars have them by writing "cars have 6 gears" instead of "some cars have 6 gears". Not something to make a big fuss out of, though.
Indeed Oda could be saving a sixth gear if he follows some cars. But it'd be weird since Luffy stated Fifth Gear to be his peak; we'll see.