RD's the guy who hooks up celebs -- like Kim, Jamie Foxx, Travis Barker, Ye and Tom Holland -- when they want a rare or luxury ride, and he was hit with a doozy of a hypothetical when he joined us Monday on "TMZ Live."

So, suppose Kim K and Holland -- who, BTW, are both featured on his new Discovery+ show -- want the same car, and there's only one available. Well, hypothetically, RD didn't even break a sweat, and quickly explained why -- in that case, specifically -- he'd give Kim the nod.

Of course, if Tom were buying the car as a gift for his GF, Zendaya, well ... that would throw a wrench in RD's plans, but he still had a solution.

"Million Dollar Wheels" shows RD wheeling and dealing with, and for, famous clients -- a job that's gotten way more challenging because now there's a lot more demand than supply.

As we've shown you, tons of celebs are tricking out cars these days -- Kanye West got a $400k custom minivan from RD -- but after 20 years as a luxury car broker, RD says Jamie is the celeb with the best feel for custom whips.