This is a flexible list. I need to start doing that also. Being self-sufficient is best, running alone on the trails is not the best thing to do, but sometimes you have to so being prepared is key.
Trail leader has final authority on trail selection for each vehicle.
When in doubt, ask the trail leader. Rating Unimproved or rarely graded road. 4WD or extra clearance needed at times, with no special driving skills required 2.5 Rating Road rarely maintained, 4WD, good clearance, low gears often needed, with some extra care and a bit of driving experience useful 3 Rating Road in difficult terrain, rarely maintained, 4WD, good clearance, and low gears essential, with some driving skill and daring required 3.5 Rating Road in difficult terrain, probably maintained only by occasional users. Excellent stock truck or utility vehicle required, with considerable driving skill and daring needed 4 Rating Trail either never bladed or badly eroded. Stock vehicles are in jeopardy.