The true weight of your vehicle and trailer can only be determined by actually weighing them on a scale. Most truck stops and some metal recycling centers have vehicle scales that can be used to weigh your vehicle and trailer.

You cannot go by published weights as these weights are not actual, but estimated. For example, most weight specifications for both tow vehicles and trailers are dry weights. A dry weight is the base weight of the vehicle/trailer without any fluids, generally without batteries, and often do not include options. When you add gas, batteries, and in some cases optional equipment such as air conditioning - even when installed by the factory, the weight of the vehicle is increased.

Both vehicle and trailer true weights can vary 20% or more over the factory "dry weight", which could result in a dangerous overloading condition if you do not allow for this difference.

The GVWR - Gross Vehicle Weight Rating is the maximum weight your vehicle or trailer can be and still be operated safely. The true weight of the vehicle, along with the weight of any passengers or cargo must NOT exceed this limit. The GVWR is typically displayed on the vehicle's placard along the "B" pillar of the cab - although some manufacturers may put them elsewhere.

Here you will find the vehicle's VIN, Tire pressure, mfg information, and the vehicle's GVWR as shown to the right.

This placard is from my Motorhome, built on a 2011 Ford E450 chassis, and the GVWR for this vehicle is shown to be 14,500lb.

It should be noted that optional equipment is often required for maximum towing, which is usually included in various towing packages. These packages may include items such as heavy duty transmissions, upgraded brakes, different gear ratios, and other equipment. For that reason, the GVWR on one vehicle of the same make and model may significantly differ from another - depending on installed equipment.

GVWR specifications for trailers are usually located on the placard near the tongue.

CGWR, or sometimes called CGVWR, is the "Combined" Gross Vehicle Weight Rating. This rating is the maximum weight of the tow vehicle, trailer, and everything in it (passengers, cargo, etc). Basically, it is the weight of the whole rig rolling down the highway. Unlike the GVWR, this specification is typically found in published towing guides of the manufacturer.

Towing Guides:
Ford Towing Guide.
Chevrolet Towing Guide.

Trailer tongue weight. This is also found by weighing. Typically, the tongue weight can vary from as little as 5% for boats to 25% or more for a 5th wheel trailer.

Environment. In the fine print of most vehicle towing guides, you will typically find a disclaimer that tow rates are determined with the assumption of using the vehicle on flat terrain and in moderate climates. If you are towing in the mountains as well as in the hot desert, your tow capacity will be less. Unfortunately, there is no published specifications as to what that limit is.

Trailer brakes. Published vehicle towing capacities also assume trailer brakes are used if the trailer is over 1,000 or 2,000lbs, depending on the vehicle. As well, most state law have a maximum trailer weight that can be towed without brakes.

Trailer hitch. Trailer hitches, as well as the ball, safety chains, and other components also have a weight rating that cannot be exceeded.

Fudge factor. I often like to add a 20% margin of safety to my towed weight. In other words, if my vehicle can tow 5,000lbs, I generally take the conservative approach and tow no more than 4,000lb. This accounts for any errors in calculating towing weights, and also any evasive driving you may have to do to avoid a collision or if you experience a flat tire.

To determine the basic maximum trailer weight you can tow, you simply subtract the GVWR from the published CGVR:

For example, my E-450's maximum trailer weight is 7,500lbs (22,000lbs - 14,500lbs). However, the factory installed hitch on my motorhome is rated at 5,000lbs, so that is my maximum towing capabilty. This is not an issue as it gives me a nice 2,500lb margin of safety. And even though the RV manufacturer's rating would be exceeded, I could tow up to 7,500lbs if I installed a 10,000lb hitch. Note that even if I install a 10,000lb hitch, I am still limited to 7,500lbs as that is the lowest of the two ratings.

To determine the maximum cargo carrying capability of the vehicle, I use a slightly different formula:

As a courtesy to vehicle owners, my motorhome manufacturer individually weighs each coach as it leaves the factory. This weight can be used for the above calculation - provided you have not installed any additional accessories.

It should be noted however that this weight is with the vehicle essentially empty - that is, no cargo or passengers are aboard, nor is there any fluids in any tanks (gas, water, sewage), and the LP tank is empty. However, as I know the GVWR and cargo capability, I can infer the actual vehicle weight of 11,559lbs (14,500lbs - 2941lbs = 11,559lbs). I will use this assumption in the remainder of this discussion - however, I should stress that it is more accurate to weigh your vehicle with it fully loaded - including passengers cargo and fluids.

The cargo carrying capacity must include:

• weight of passengers.
• weight of cargo.
• weight of any additional installed equipment.
• weight of a full load of water (8.3lbs per gallon).
• weight of any fluids in the black and grey holding tanks.
• weight of a full load of fuel (7lbs per gallon).
• weight of a full LP tank (typically 30~50lbs).
• weight of the trailer tongue. For a 5,000lb trailer, a 10% tongue weight would be 500lbs.

A common practice many RV'ers employ when traveling long distances is to dump all of the holding and water tanks. Traveling with full tanks could weigh 500lbs or more, so this can be a significant amount of weight to carry - and could change the handling characteristics and fuel mileage of the coach.

You can figure out the cargo carrying capacity of the trailer in a similar manner. If a boat trailer has, say a 4,500 GVWR, and the boat itself weighs 3,500lbs. you can add a maximum of 1,000lbs of cargo to the boat. However, you must not exceed the GVWR rating of the trailer nor the maximum trailer towing capacity of the towing vehicle.

You may safely tow your rig only if all of the weights are under the maximum allowable, which includes the vehicle's GVWR, the trailer's GVWR, the vehicle's CGVR, the hitch rating, and the maximum trailer weight allowed by your state and owner's manual (for trailers without brakes).

One important fact is that you cannot increase the coach'es cargo carrying capability by not towing a trailer (other than the tongue weight) or towing a light trailer. You can however, off-load some of the cargo from the coach into the towed trailer - as long as you do not exceed the cargo capability of the trailer.

When selecting a tow vehicle, keep the follwing in mind:

• Typically the more weight of the vehicle, the less it can tow (providing the engine, transmission, and gearing is the same). A standard cab 4x2 pickup can likely tow more than a crew cab 4x4 as it weight less.
• Higher gear ratios typically tow more, but gas mileage is usually reduced.
• Most non-commercial vehicles require additional equipment to tow the maximum, including a heavier duty radiator and/or transmission cooler, tires, brakes, and suspension components. These are often included in a tow package.
• Don't assume your tow vehicle has the maximum allowable towing ratings for your model, unless you know for sure it has the required optional equipment.
• Tow ratings are generally calculated for flat terrain and in moderate temperatures.
• Actual vehicle and trailer weights can differ significantly from published dry weights.
• Always consider reducing your towing capacity by 20% to provide a cushion of safety.
• Tow at a reduced speed when on highways, especially when towing a trailer with smaller tires. 70MPH is not a safe towing speed.

Resources:

Water weighs 8.3 lbs per gallon.
Gasoline weighs 7 lbs per gallon.
Class I hitches (light duty) have a limit of 2,000lbs.
Class II hitches (medium duty) have a limit of 3,500lbs.
Class III hitches (heavy duty) have a limit of 5,000lbs, and may require weight distribution.
Class IV hitches (extra heavy duty) have a limit of 10,000lbs and must be either weight distributing or 5th wheel.
Class V hitches (maximum duty) can have loads above 10,000lbs and must be weight disributing or fifth wheel.