The measurement is only taken when the other three tires are still on the ground. The distance traveled up the ramp is then measured and is divided by the vehicle's wheelbase and finally multiplied by 1000 to give a final RTI score.
A specific measurement procedure is followed: all non-driving tires must be in contact with the ground. The distance traveled as the vehicle climbs is then calculated, divided by the distance between the driving wheels, and finally scaled up by a factor of 1000 to determine an RTI rating.