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