Louisville vs. Kentucky -- Pistons Teammates Wager: Loser Wears My School's Gear

Regardless of who wins the Louisville-Kentucky NCAA tourney game tonight ... somebody's gonna look pretty damn foolish in the Detroit Pistons locker room.

Peyton Siva and Josh Harrellson -- who are now teammates in Detroit -- were arch rivals back in 2010 ... when Siva played for Louisville and Harrellson for Kentucky -- two schools that HATE each other.

Siva and Harrellson have been able to bury their true feelings for the sake of their team -- until now ... telling TMZ Sports they've got a friendly bet going over tonight's Sweet 16 grudge match.

Siva says "We have a gear bet ... I have some gear for him [to wear after we win!]"

The opposite holds true for Harrellson ... but Siva wasn't eager to discuss that outcome.

Siva says he and Harrellson will be watching the game on mobile devices ... since they'll be on a Pistons flight headed to Philadelphia.