Steven Tyler's Topless Joyride in Hawaii -- How Many Naked Chicks Can You Count?

BOOB ALERT -- if you didn't already know it was Steven Tyler in this video ... you might think there were two topless chicks cruising around in the back of a Jeep Wrangler like drunk high schoolers.

The person who shot the footage tells TMZ, he honestly believed he was being tailed by a car with two topless women -- until the Jeep got close enough for him to see ... it was actually Steven.

It's unbelievable. You can even hear Steven singing at the top of his lungs.