Reading this thread everyone alludes to the same factors that effect mileage when towing:
Trailer Weight
Aerodynamics (width or resistance)
Aerodynamics (shape or drag)
Tow Speed (breakdown of aerodynamics)
Missing is the relationship of the horsepower, torque and aerodynamic efficiency of the tow vehicle.
Many vehicles are optimized to provide the proper horsepower and torque to maintain the best possible fuel mileage at a certain weight and drag. One of the posters has a Golf TDI, this is a highly optimized diesel that get great mileage; when you operate it a certain way. A couple of years ago I went on a trip with friends. We budgeted out money to share the expense of a long drive across Germany and France. We took out funds at the bank to cover the whole trip before we left to avoid ATM fees on the road. Then we met a couple of Americans backpacking across Europe who need to get to Frankfurt to return home to deal with a family emergency. No problem we were heading to Frankfurt anyway and they had plenty of time to make their flight. The problem, the additional weight of our passengers and gear (about 350 lbs by my estimate) total threw off the fuel mileage. The finely setup TDI dropped 8-10 MPG! We were driving on the same roads at the same speeds! I did notice we were: downshifting earlier on inclines, and lost some aerodynamic efficiency as the front end was slightly higher. I also noticed we were spending more money on fuel and ATM fees. But how could the addition of two passengers and a couple of backpacks cause so much havoc to the mileage?
The additional weight and loss of aerodynamics threw the diesel engine outside its most efficient operating range.
This is why cars with small engines and great gas mileage loose so much efficiency when adding a trailer.
They have the perfect weight to power and torque ratio until you add the trailer.That's my 2 cents worth,
-TechScott
Mission, Texas USA