THE TRAIL RATING SYSTEM
The Rating System

Author:
Well, for one thing, the rating system is very subjective and this one, though used a lot, doesn’t happen to be to only one used. In other words, it isn’t like the Yosemite Decimal System that is now used across the USA for rating rock climbs. Every state has a 4x4 rating system that varies to some degree and even certain areas have different ratings in the same state.
This will only get you in the ballpark and certainly doesn’t take in the weather conditions.

Colorado
Trail Rating System
System used by the Colorado Association of 4 Wheel Drive Clubs, Inc. (CA4WDCI).
This system has been adopted by many others.
1 Graded dirt road. Dry, or less than 3" water crossing depth. Gentle grades. 2WD under all conditions except snow. No width problems, two vehicles wide.
2 Dirt road. Dry, or less than 3" water crossing depth. Some ruts. Slight grades, up to 10 degrees. 2WD under most conditions. Rain or snow may make 4WD necessary. Usually one and a half to two vehicles wide.
3 Dirt road. Rutted, washes, or gulches. Water crossings up to 6" depth. Passable mud. Grades up to 10 degrees. Small rocks or holes. 4WD recommended but 2WD possible under good conditions and with adequate ground clearance and skill. No width problems for any normal vehicle. Vehicle passing spots frequently available if less than two vehicles wide.
4 Rutted and/or rocky road. No shelves but rocks to 9". Water crossings usually less than hub deep. Passable mud. Grades moderate, up to 15 degrees. Side hill moderate up to 15 degrees. 4WD under most conditions. No width problems, vehicle passing spots frequently available if less than two vehicles wide.
5 Rutted and/or rocky road. No shelves. Rocks up to 12" and water crossings up to 12" with possible currents. Passable mud. Moderate grades to 15 degrees. 6" holes. Side hill to 20 degrees. 4WD required. No width problems.
6 Quite rocky or deep ruts. Rocks to 12" and frequent. Water crossings may exceed hub depth with strong currents. Shelves to 6". Mud may require checking before proceeding. Moderate grades to 20 degrees. Sidehill may approach 30 degrees. 4WD necessary and second attempts may be required with stock vehicles. Caution may be required with wider vehicles.
7 Rocks frequent and large, 12" and may exceed hub height. Holes frequent or deep (12"). Shelves to 9". Mud 8" deep and may be present on uphill sections. Grades to 25 degrees and sidehill to 30 degrees. Water crossings to 18" and may have strong currents. 1-1/2 vehicles wide. 4WD required. Driver experience helpful.
8 Heavy rock and/or severe ruts. Rocks exceeding hub height frequent. Shelves to 12". Deep mud or uphill mud sections. Steep grades to 25 degrees and can be loose or rocky. Water crossings may exceed 30" in depth. Side hill to 30 degrees. One vehicle wide. Body damage possible. Experience needed. Vehicle Modifications helpful.
9 Severe rock over 15". Frequent deep holes over 15". Shelves over 15". Mud bog conditions (long, deep, no firm bottom). Over 30" water crossings with strong currents. Steep grades over 30 degrees. Sidehill over 30 degrees. May not be passable by stock vehicles. Experience essential. Body damage, mechanical breakdown, rollover probable. Extreme caution required.
10 Severe conditions. Extreme caution recommended. Impassable by stock vehicles. Winching required. Trail building necessary. May be impassable. Impassable under anything but ideal conditions. Vehicle damage probable. Personal injury possible. Extreme caution necessary.
Note: Trail ratings are highly subjective. A trail that is easy for an experienced driver with a highly modified vehicle may be impassable for a novice driver in a stock vehicle. Weather conditions may also significantly affect the difficulty of a trail. Also, the difficulty of a trail may change over time, as trail use, weather, and maintenance modify the trail. Always check with the managing organization and/or recent users before planning a trip on a trail and scout ahead on foot if you are not sure of current conditions.

This site also lists various ratings systems for other parts of the country:
http://www.4x4trailinfo.com/ShowRating.php?system=5