Ryan Lochte -- Shows Off Big Tip in Vegas

Olympic gold medalist and failed reality star Ryan Lochte still has tons of cash -- or maybe just sucks at math -- because he just dropped a 200% tip at an expensive Las Vegas steakhouse ... TMZ has learned.

Sources at Andiamo Italian Steakhouse in the D Casino & Hotel tell us, Lochte stopped by with a bunch of dudes Wednesday night and ran up a $314 tab -- on top of which Lochte added gratuity of $606, for a grand total of $920.

We're told Ryan -- whose reality show "What Would Ryan Lochte Do?" was canceled after one season -- asked all the waiters to split up the ridiculously generous tip evenly.

But we know ... you're still wondering why Lochte's pockets are so damn fat?

Our sources say he had just won big at the roulette table before dinner and was feeling pretty flush. Ain't no money like house money!