We haven't found a definite original source for this system, but have seen it used mostly on Arizona trails and web sites and TTORA sites.
|1.0||0||2WD - All-weather high speed paved or graded dirt or gravel road, regularly maintained, with excellent drainage. 4WD not needed, even in wet weather. Speeds of over 40 mph can be achieved. Passenger cars acceptable.|
|1.5||1-2||2WD - Maintained graded dirt road. 4WD not needed except in wet weather. May have potholes, dips, and sharp curves. May drain poorly in wet weather. Speeds of over 10 mph possible. Passenger cars can do, but will have to slow down considerably on washouts, etc., due to lack of ground clearance. No grades over 10 degrees.|
|2.0||2-3||Easy 4WD - Unimproved or rarely graded dirt road; 4WD and/or extra clearance occasionally needed at times, with no special driving skills required. Passenger cars NOT acceptable, due to insufficient ground clearance. 2WD & 4WD trucks with stock clearance are acceptable. Street tires are acceptable. Day runs are typically 20 miles in length.|
|2.5||4||Easy 4WD - Dirt road graded rarely, if ever. 4WD low range and good clearance often needed, with some extra care and a bit of driving experience useful. Suitable for novice drivers. Speeds often under 10 mph. Street tires are acceptable. 2WD trucks NOT acceptable, due to lack of low range gears. No grades over 20 degrees.|
|3.0||5||Moderate 4WD - Ungraded road in difficult terrain, rarely maintained. 4WD low range and clearance required, with some driving skill and daring useful. Challenges for novice driver. Suitable for stock 4WD vehicles. Street tires are not recommended. No 2WD vehicles due to lack of traction. Speeds often under 5 MPH.|
|3.5||6-7||Moderate 4WD - Road or trail in difficult terrain, probably maintained only by occasional users. Considerable driving skill and daring needed. Not recommended for the novice driver. Surface may include wash beds, sand, cinders, lava rock, snow, mud, water, etc. Vehicles with long front or rear overhang (beyond the axles) may scrape front or rear bumpers on wash crossings, etc. Street tires not acceptable. Approaching the limit of what a capable stock 4WD vehicle (with capable driver) can do. Stock 4WD vehicles usually not recommended. Upgrade to 31" tires, mild lift (1" or 2"), and rear locker or limited slip is highly desirable. Equipment failure is usually limited to flat tires. Grades may exceed 20 degrees. Occasional off camber situation may exceed 20 degrees. Water crossings should be 2 feet or less in depth. Spotting may be required on the more difficult sections, especially for long, wide vehicles. Average speed is 3 mph. A day run is typically around 10 miles in length. This is the most common numerical rating given to 4WD trails. Suitable for mildly modified 4WD vehicles & moderately experienced drivers.|
|4.0||8||Hard 4WD - Trail badly eroded. Off camber situations likely and may exceed 25 degrees. Grades may exceed 25 degrees. Water crossings can exceed 2 feet in depth. Stock vehicles NOT recommended. 31" tires minimum, (33" preferred), mild lift & hard locker, (not limited slip), in the rear probably required. Front locker or limited slip recommended. Speeds often under 3 mph. Spotting probably required on the more difficult obstacles to avoid body damage. Recommended for experienced 4WD drivers only. Body damage possible, but unlikely. More likely on long, and/or wide vehicles. Equipment failure possible, but unlikely.|
|4.5||9||Hard 4WD - Trail is TOUGH. Big rocks, grades exceeding 30 degrees. Off camber situations exceeding 30 degrees. Sometimes, but not always, best suited for short wheel base vehicles. (under 105" wheelbase) 33" tires minimum, (35" or larger preferred). 2" to 4" of lift and gearing of 40 to 1 (20 to 1 for automatics) or lower in first gear, low range probably required. Hard locker in the rear and limited slip or hard locker in the front probably required. Speeds often under 1 mph. A day run will usually be 1-3 miles in length. Body damage likely, but usually can be avoided by very careful driving and good spotting. Typically 10% of the vehicles on these runs will experience major mechanical failure, i.e., drive line or steering failure. (u-joints, axles, pitman arm, drag link, tie rods, etc.) Usually, but not always, these failures are due to improper and insufficiently tested vehicle modifications or poor (sloppy) driving technique. (Too much gas or steering pressure) An experienced driver with a properly setup vehicle can usually avoid most breakage.|
|5.0||10||Hard 4WD - Trail CANNOT be driven the whole way. Winching is required in at least one point on the trail. Water crossings may exceed 2-? feet in depth. Grades exceeding 40 degrees. The biggest rocks with off camber situations exceeding 35 degrees. Rollovers possible. Usually only suitable for short wheel base vehicles (under 105" wheelbase), winch, 33" minimum tires, (35" or larger preferred), 3" to 5" of lift, gearing of 60 to 1 (30 to 1 for automatics) or lower in first gear, low range, and hard locker in both front and rear axles probably required. Axle upgrades highly recommended. Speed continuously under one mph. A day run is typically one mile in length. These trails are typically very hard on tires. One spare probably required, two spares recommended. Plug kit and/or tubes recommended. Tires with sufficient sidewall protection (Boggers, Swampers, etc.) recommended. 50% or greater chance of body damage. 50% or greater chance of major mechanical failure. Breakage is usually due to excessive torque or torque spikes on axles or steering components. Only world class, experienced (5 years +) drivers with extra money, parts & energy to repair broken vehicles need apply.|
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.