[IWillTry]

If, for whatever reason, you want to measure Crr more accurately, without including drivetrain friction and viscous forces here is one method.

Use a spring scale to measure the force to pull your vehicle on flat ground at a constant speed (a walking pace). Call this force F1. Then jack up each wheel in succession, wrap a cord around the outside of it several times and use a spring scale to measure the force required to spin it at constant speed (walk away at the same walking pace as before). Add up the forces for all 4 wheels to get F2. Then Crr = (F1-F2)/Vehicle_Weight.

For even more accurate results, measure both drive wheels simultaneously (requires 2 spring scales and two people walking at the same pace).