Luffy at present strength with all his gears V.S. Sir Crocodile.
Luffy has no knowledge about Sir Crocodile's abilities whatsoever, neither does Crocodile know about Luffy's.
The two fights.
Luffy goes gear 2nd, punches 20000 holes in Croc.
Luffy goes gear 3rd, punches a giant crater.
Luffy goes: "WTF??"
Crocodile turns Luffy into mummy.
Battle is over.
Basically back in Alabasta, during the first Luffy V.S. Crocodile, even if Luffy has his current power level, he still would lose. Luffy only managed to learned Croc's weakness by sheer chance.
The above scenario applies to current Luffy against Smoker as well. Smoker is probably the weakest Logia user thus far. He has no mastery over his devil fruit and smoke doesn't exactly sound as scary as fire or lightning. But Luffy still would lose against Smoker.