To determine a rough estimate of horsepower, input car weight including the driver's weight and actual quarter mile time.
Most accurate results for flywheel horsepower depend on the accuracy of the user input. Using actual weight and 1/4 mile time is best.
The resulting number is the engine horsepower at the flywheel which is the measurement car manufacturers claim. There is a loss of horsepower through drivetrain. Many sources say that you can expect at least a 15% loss of engine horsepower with front and rear wheel drive cars, more for all wheel drive vehicles. Efficiency is a major role in reducing drivetrain loss, many times what improves gas mileage also improves wheel horsepower through mechanical efficiency.
What is horsepower? Horsepower is a measure of performance based on the weight of coal one horse could pull. One horse could pull approximately 330 pounds 100 feet in the air in one minute using a pulley system. The product for 1 horsepower will always be equivalent to 33,000 foot-pounds in one minute.