Yordano Ventura Crash ... Investigators Believe He Was Speeding

Transportation authorities in the Dominican Republic tell TMZ Sports they believe Yordano Ventura was speeding at the time of his fatal crash.

The K.C. Royals pitcher was traveling on a mountainous highway around 4 AM on Sunday when he lost control and flipped his customized Jeep ... killing him in the process.

We're told the speed limit in the area is 25 kph ... which is only about 15 mph ... and a local official tells us they believe there's evidence showing he was over the limit.

Officials have previously said Ventura was driving through a foggy area and over-corrected when his tires went off the road. He was not wearing a seat belt at the time of the crash and was thrown from the vehicle.