Paul Walker liked his cars fast fast fast -- in fact, he once boasted about hitting a top speed of 185 MPH -- and it wasn't on a racetrack ... it was on a freeway.

"The Fast & The Furious" star -- who died in a fiery car crash on Saturday -- made the revelation on GTChannel back in 2011 ... while taking a Nissan GT-R out for a test spin in Japan.

"The fastest I've ever gone is 184 to 186," Walker says on the video .... and then he reveals how fast he was clocked on an actual racetrack.