Here is a site to help calculate towing capacity.
Read more: http://www.ehow.com/how_4815339_calculate-towing-capacity.html#ixzz2ymgRtdtR
Instructions
1
Locate the vehicle data tag on the driver's door post.
2
Note the CGWR. This is the combined gross weight rating. It is the maximum allowable weight your vehicle, with cargo and passengers, and a trailer can weigh.
3
Determine the loaded weight, with passengers and fuel, of your vehicle. This is most accurate by weighing the vehicle on a public scale. These can be found at truck stops and some moving companies. Check also at landscape supply companies and sand and gravel yards. If a scale cannot be found, estimate the weight by adding the curb weight of the vehicle and all passengers and cargo.
4
Subtract the loaded weight of your vehicle from the CGWR rating of your vehicle. The resulting number is your towing capacity. Keep the weight of your trailer under this number to be safe and legal. Weighing your trailer when fully loaded is most accurate.
My personal 2 cents.
If you have a Sprinter 3500 chassis the tow capacity is suppose to be 5000 lb.
However I would never tow more than half that weight. After calculating the tow capacity I stay below the maximum capacity, to provide a safe margin for those unknowns.
Why; as stated above the transmission isn't designed for continuous operation at the maximum. Stress on the engine, brakes, chassis, suspension and fuel mileage will be considerably lower. If you're towing on flat Oklahoma paved road may not be to bad, but take that to Colorado or Utah Bryce is at more than 8000 ft. Can you imagine the stress on the vehicle. Also consider road surface is it paved rolling drag isn't to high but take it off the pavement on that dirt road (sand, gravel, bumps) what is the drag?
Additional weight; what will you put in the vehicle being towed that adds up, looks like a good place to put extras.
Most people never consider the variables just assume that the book says 5000 lb so should be Ok, I don't think so.
S Bradley
Navion IQ 24V