The Big Three's New Diesels put on a Dyno

Dynos measure torque and calculate horsepower based on the torque reading at a certain rpm.

Super Chevy wrote:
"What dynos actually measure is torque, not horsepower," explains Roberts. "Since horsepower is a mathematical equation, the dyno measures torque then calculates horsepower."

Learn About Dyno Testing - How Dynos Work