Estimate fuel with a trailer.
Towing costs fuel through two mechanisms: the extra mass to accelerate and haul up gradients, and the aerodynamic disturbance of a box behind the vehicle. Combined, the observed penalty is roughly 5% of economy per 1,000 lb of trailer for a conventional box or caravan — so a 5,000 lb trailer takes about 25% off, turning 20 mpg into 15. Tall, flat-fronted trailers sit at the worse end of that range and low aerodynamic ones at the better. Note the compounding effect on range: at 15 mpg instead of 20, the same tank goes a quarter less far, which changes where you can plan to stop as much as it changes the cost.
Towing Fuel Cost
Economy loss% = trailer weight / 1000 x 5; towing mpg = unloaded mpg x (1 - loss%); fuel cost = distance / towing mpg x fuel price
Never exceed the tow rating, gross combination weight or tongue weight limits in your vehicle handbook. Overloading affects braking and stability and may be illegal as well as dangerous.
Roughly 5% per 1,000 lb of trailer, so 20-30% for a typical 4,000-6,000 lb caravan and up to 50% for a large travel trailer. Frontal area matters as much as weight.
Substantially. Dropping from 65 to 55 mph often recovers 10-15% of economy while towing, because the trailer's aerodynamic penalty grows with the square of speed.