Joe Morton would love to reunite with Sandra Bullock and Keanu Reeves for a 3rd "Speed" movie ... but isn't yet sold on the plot, TMZ has learned.

The actor tells TMZ ... he'd love to be a part of "Speed 3," after Sandra and Keanu both expressed an interest in rebooting the franchise. However, Joe -- who played Lieutenant Herb 'Mac' McMahon in the OG -- says there are some big questions that need to be answered before he signs on for any new movie.

He adds ... "'Speed' was a wonderful roller coaster ride; the bus had to careen through the city and jump a gap in a bridge and couldn’t go below 50mph or it would blow up -- what’s 3’s vehicle?"

As Joe puts it ... the vehicle is the key to the film's success -- calling out how the boat in "Speed 2: Cruise Control" didn't really work.

Remember, even Sandra slammed the follow-up flick, telling TooFab in 2022 ... "I've been very vocal about it. Makes no sense. Slow boat. Slowly going towards an island."

Vehicle questions aside ... Joe says there's a lot that needs to be addressed if a 3rd film were to happen.

He continues ... "Keanu’s character was involved because he foiled Dennis Hopper’s plan to blow up a building. Who’s the new villain? Is Jack still a cop? What’s the villain’s connection to Keanu?"

Joe saves his most important question for the end ... asking if Sandra and Keanu's characters are romantically involved heading into "Speed 3." FWIW, we're wondering the same thing!!!

The actor suggests looping in original screenwriter Graham Yost so that "Speed 3" can actually top "Speed."