And from RV world.
Auto manufacturers base their towing capacities on a vehicle's base curb weight. Optional equipment is not included in this weight. This figure is of little use when trying to determine your payload. Knowing your actual payload capacity will keep you from exceeding your gross vehicle weight rating(GVWR).
To determine your available payload, first weigh your tow vehicle(TV), unless you have an accurate delivery weight. With the actual weight figure, add the weight of your family including yourself, a full tank of fuel, and the cargo you would carry in your TV. Subtract that total from your GVWR and you now have the remaining available payload (what is left for tongue weight).
The maximum TW you can add to your TV will depend on whether or not you use a weight distribution(WD) hitch, sometimes called an equalizing hitch. A WD hitch removes some of the load from the TV's rear axle and distributes it to the TV's front axle and the TT's axle(s). Check with your manufacturer to be sure that you can use a WD hitch as it puts stress on the TV's components.
If you do not or can not use a WD hitch, all of the TW gets applied to your remaining payload capacity. If you exceed this, then you have exceeded your GVWR. Furthermore approx. 150% of the TW will be added to your rear axle(the extra 50% is front axle weight distributed to the rear axle). This could exceed your rear gross axle weight rating(GAWR). The load on your front axle will be reduced by 50% of the TW and both steering and braking would be compromised. Be aware of the hitch ratings on your TV. There will be a lower rating when not using a WD hitch. Always be sure that your tires are rated for your GVWR at proper inflation.
If you use a WD hitch and utilize appropriate spring bar tension(SBT), then you can expect to apply only about 67% of TW (+ the weight of the WD hitch itself) to your payload capacity. Total SBT in pounds should be equal to double your tongue weight. Approx. 33% of the TW is distributed to the TT axles(s) and the load on the receiver will be reduced by approx. 33%. The load on the rear axle of your TV will only be about 50% of the TW.
TV base curb weight - 5546#s
TV fully loaded - 6500#s
GVWR - 7200#s
Remaining payload capacity - 700#s (GVWR - 6500#s)
GAWR (rear) - 4000#s
Receiver Cap. - 500#s weight carrying / 1000#s WD
SBT - 2000#s total
WD Hitch - 100#s
Tongue Weight of your dream TT - 900#s
Without a WD hitch the 900# TW will overload the remaining payload capacity (700#s) and the GVWR (7200#s) by 200#s. The receiver hitch rating is overloaded and the rear GAWR will probably be overloaded as well as there would be an increase of 1350#s on the rear axle.
With a WD hitch, the TW will increase by the weight of the hitch itself to 1000#s. This would put 670#s(67% of TW) onto the TV and be just under the remaining payload capacity. The hitch capacity rating is now 1000#s, so you are OK there as well. There will only be a 500# increase on the rear axle(50% of TW). About 170#s (17% of the TW) will be on your front axle, as steering and braking dynamics are much less affected.
Using a WD hitch in this example would keep you inside of your capacities, but towing performance might not be great. Towing performance is somewhat subjective, but most members recommend that you tow at 80% of your capacities. It is very important to take into consideration the engine size and rear axle gear ratio. Towing near 100% of your capacities can lower the normal lifespan of your engine, transmission, and other components. It is always best to take actual weight readings of your TV, TT and individual axles. It is also possible to exceed the gross combined weight rating(GCWR) and not exceed the GVWR.
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manufacturers generally base their trailer weight and tongue weight on a base unit with little or no options. They call it the unloaded vehicle weight(UVW) and will provide the dry TW or dry hitch weight. To get your actual TT weight add the weights of all options not included in the UVW, full propane, weight of battery, and weight of a full fresh water tank. To get the actual loaded TT weight you will need to estimate the weight of your gear and food. The actual loaded TW will generally be between 10% and 15% of the loaded TT weight depending on the manufacturers axle placement and how you load it.
Edited 9/23/04: The above results using a WD hitch represent an attainable goal. The weight distributed back to the TT axle(s)is dependent on adequate spring bar tension, the length of the spring bars, the distance from the hitch ball to the spring bar attachment location and the distance from the hitch ball back to the TT axles. These variables will change the end result. Other factors such as wheelbase and rear overhang will affect how the TV axle weights are distributed.