The weight of the trailer is the most important factor. A good weight-to-horsepower ratio is about 35:1; that is, 35 lbs. of Gross Combined Weight (GCW) for every horsepower your engine develops. Multiply the engine's rated horsepower by 35 and subtract the combined weight of the vehicle, passengers and luggage. The number remaining is the approximate maximum weight you should tow, although a numerically higher axle ratio may help compensate for heavier weight.