Wal-Mart Stores finally hit its number this morning, posting its first quarterly gain in store traffic in 24 months. This may be cause for other retailers to celebrate as well, as Wal-Mart welcomed cash that would have otherwise been spent on gas. It’s a theory popular among economists: Plummeting fuel costs lift all boats, not like a tide but like a sort of shipping lock, to play the metaphor through. Analysts at Goldman Sachs note that the longer gas prices stay low, the more consumers like to spend. There’s some evidence that’s exactly what happened at Wal-Mart in the recent quarter . Revenue from the fuel pumps at the company’s Sam’s Club stores was down 2.4 percent in the quarter. Meanwhile, sales at Sam’s Club minus fuel costs were up 2 percent. Companywide, Wal-Mart revenue ticked up 1.5 percent. U.S. consumers spend about $370 billion a year on gasoline. […]