Tow Vehicle Sizing
Proper sizing of the tow vehicle is a critical component of safe towing. Most often this vehicle is a full size pickup truck, but properly equipped cars, vans, SUV’s, and even large motorcycles, are capable of towing small trailers.
On this page we provide the steps for determining the vehicle ratings for safe trailer towing. By “safe” we mean that none of the weight ratings of the tow vehicle have been exceeded.
Not only should the ratings be strictly adhered to, it is advisable to leave a safety margin. The amount of safety margin varies with the person suggesting it, but a commonly agreed upon minimum is 20%.
Each vehicle has numerous weight ratings, or restrictions, as determined by the manufacturer. When towing, we are not only concerned about the vehicle and RV, but also the additional weight that the trailer will place upon the vehicle.
Before we look at the methods for determining the tow vehicle ratings, let’s look at some weight definitions:
Gross Vehicle Weight (GVW) is the actual weight of the fully loaded vehicle, including all cargo, fluids, passengers, and optional equipment, as measured by a scale. GVW is determined by weighing the entire vehicle, without the trailer attached.
Gross Vehicle Weight Rating (GVWR) is the maximum number that the GVW should never exceed. In other words, this is the most that the tow vehicle should weigh.
Rear Gross Axle Weight (RGAW) is the actual weight placed on the rear axle. Assuming a well-balanced vehicle, the RGAW is evenly distributed to all tires on that axle. RGAW is determined by weighing all wheels on the rear axle, without the trailer attached.
Rear Gross Axle Weight Rating (RGAWR) is the maximum number that the RGAW should never exceed. In other words, this is the most weight that the tow vehicle’s rear axle should carry.
Gross Combination Weight Rating (GCWR) is the maximum number that the tow vehicle GVW plus the trailer GVW should never exceed. In other words, this is the most that the entire RV should weigh. This includes the tow vehicle, trailer, anything towed behind the trailer, and everything inside and on top.
Maximum Tongue Weight or King Pin Weight
The Maximum Tongue Weight and Maximum King Pin Weight is the most weight that should be pressing down vertically on the tow vehicle hitch.
Maximum Trailer Weight
Maximum Trailer Weight is the heaviest trailer that the vehicle should tow, as determined by the manufacturer.
Hitch Rating is the most weight that a hitch can support vertically or pulling horizontally. This rating may be specified by the vehicle manufacturer and the hitch manufacturer—if they are different, use the smaller number. For ball mount hitches, this rating refers to the weight pressing down vertically. For fifth wheel hitches, this rating refers to the entire weight of the trailer.
Look Up Tow Vehicle Ratings
The first step in sizing a tow vehicle is to look up its manufacturer ratings. Some ratings may be printed on stickers on the vehicle itself. In the case of pickup trucks, stickers are usually affixed to one of the door jambs or to the cab interior near the back.
For a complete list of ratings, consult the documentation for the specific vehicle in question, available at dealers and on manufacturer web sites. Common names for these documents are towing guide, trailering guide, or vehicle specifications. We have compiled links for the most popular trucks on our Truck Ratings for RV Towing page.
Determine Tow Vehicle Weights
Next, we need to determine the actual weights of the tow vehicle. This information is crucial to accurately determine towing capacity. To weigh the tow vehicle, we configure it with all options and cargo, as if ready to tow the trailer. Then, we measure the following weights on a truck scale, without the trailer attached:
- GVW: Weight of the entire vehicle. If the wheels are weighed individually, GVW is the total of all wheels.
- RAWR: Weight carried by the rear axle. If the wheels are weighed individually, RGAW is the total of all wheels on the rear axle.
If the vehicle did not contain all options and cargo when weighed, then we need to adjust GVW by adding all missing weights, as accurately as possible. For example, if we weighed the truck with the driver only, but a spouse and two kids will be traveling with the RV, then we need to add the weight of the family to the GVW. To make this calculation easier, we created the Adjust GVW Calculator.
Determine Towing Capacity
The final step is to subtract the actual weights from the ratings, leaving the remainder as the towing capacity. It is important to consider all calculations below and use the smallest, or most restrictive, for determining the maximum trailer weight the vehicle should tow.
GCWR – GVW = Maximum Trailer Weight
The absolute maximum that the trailer should weigh is the difference between GCWR and GVW. If the manufacturer-specified Maximum Trailer Weight is lower, then it takes precedence.
RGAWR – RGAW = Maximum Tongue or King Ping Weight
Because almost all of the tongue weight or king pin weight is supported by the rear axle, the difference between RGAWR and RGAW is the most tongue or king pin weight the vehicle can support.
If the RGAWR – RGAW formula produced a larger result than the hitch rating, we must use the more restrictive hitch rating instead, as the maximum tongue or king pin weight.
Once the maximum tongue or king pin weight has been determined, we can work backwards to determine the trailer weight. For travel trailers, the maximum recommended tongue weight is between 10 and 15% of the trailer weight. For fifth wheels, it is 15 to 25%. Knowing this, we calculate the maximum trailer weight as follows:
Max. Trailer Weight = (max. tongue or king pin) ÷ percentage
For example, if we calculated the maximum tongue weight as 1,200 lbs, and assume a 15% ratio, the formula will look like this:
Max. Trailer Weight = 1200 ÷ 0.15 = 8,000 lbs
And at a 10% ratio:
Max. Trailer Weight = 1200 ÷ 0.10 = 12,000 lbs
GVW + Maximum Tongue or King Pin Weight ≤ GVWR
We make sure that the maximum tongue or king pin weight does not push the vehicle over the GVWR. We do this by adding the maximum tongue or king pin weight calculated in the previous section.
Weight is the paramount concern when sizing a tow vehicle, but certainly not the only consideration. The heavier the trailer, the more useful the following features become:
- Long wheelbase: Helps to keep a straight course.
- Weight-distributing hitch: A ball hitch system that distributes some of the tongue weight to all axles of the tow vehicle and trailer. This provides more weight on the front axle for better steering control, and less weight on the rear axle to allow towing a heavier trailer.
- Anti-sway bars: Provides greater lateral stability. Helps distributes weight to four wheels, allowing truck to carry more tongue or king pin weight.
Online Weight Calculators
DISCLAIMER: While we believe that all information on this web site is accurate, we can not guarantee that it is applicable to You specifically or to Your situation. More »
While it is important to understand the formulas involved in sizing a tow vehicle, repeated calculations by hand become cumbersome.
The following calculators automate the tasks discussed above:
Andy Herrick is a blogging nerd, #8 Enneagram, wannabe bread baker, INTJ, RV industry professional, and small business entrepreneur. He can be found hanging out with his lovely wife and family, skiing, cycling, climbing, hiking, and convincing anyone who will listen why dogs aren’t really that great of pets. Also, he runs this website.