The Legal Side
Having your tow vehicle properly equipped is the biggest factor in towing safely. It's also the biggest factor in avoiding serious legal and possible financial woes: Failure to have your vehicle and trailer properly equipped places you at great liability risk in the event of an accident where injuries occurred under what the legal system calls the Law of Negligence.
If the vehicle's owner's manual uses words such as "requires," "must have," or "not to exceed" in describing certain weights, limitations, and driving instructions related to towing and you ignore those caveats/instructions, you could be held liable for damages in a lawsuit brought against you by the injured party.
Towing Speed and Fuel Economy
Safety isn't the only good reason to slow down. Your wallet will appreciate it, too. According to the EPA, tests designed to imitate highway driving reveal that 54 percent of a tow vehicle's engine power is used to overcome aerodynamic drag. If you drive faster, the engine has to work even harder to push through the air, and it consumes more fuel doing so. A good example of how drag affects fuel economy is a truck that has an 18-mpg highway EPA number, which is based roughly on 65-mph speeds. Drive 70 and drag causes that fuel economy to fall about 1.5 mpg. Run at 75 and your mileage could drop another 1.5 mpg.
Put a trailer on the hitch ball and now fuel economy and drag play a bigger role. When the gross weight of the trailer is more than half the weight of the tow vehicle, the added weight and surface area moving through the wind can easily reduce your tow vehicle's fuel economy by 40 percent. As trailer weight goes up, fuel economy drops even further. If your tow vehicle gets 17 mpg at 70 mph in everyday use, it may get only 12 mpg with a small travel trailer or tow hauler in tow. Slow down to 60 mph and that mileage could easily jump two mpg.