From Wikipedia the Ramp travel index or RTI, is a way of measuring a vehicle's ability to flex its suspension, a property also known as axle articulation. The RTI rating is used mainly in the off-roading industry to test and describe chassis limits of modified vehicles.

First Step
Measure your vehicle’s wheelbase which is the distance between where your rear and front wheels touch the ground. Any units such as inch or centimeters will do. We will call this measurement D. See Illustration.

Second Step
Jack up you Vehicle’s front wheel to the limit before lifting a rear wheel. Use a high-lift jack, forklist or drive up something:

Third Step
Measure the distance between the bottom of your raised wheel and the ground. Make sure you measure a line (not necessarily vertical) to the ground opposite the bottom of the other front wheel along the axle. Use the same units as before. We will call the measurement H. See illustration above.

Fourth Step
Plug in your D and H numbers below and press the STI score button to get your score

Distance from Ground (H):
Wheelbase (D):