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A power-to-weight ratio refers to how much horsepower a vehicle has in relation to its weight. When factoring a power-to-weight ratio, the weight of a vehicle must be divided by the number of horsepower the engine produces. This measured power-to-weight ratio can be applied to any type of engine-powered vehicle, from a motorcycle or automobile to a locomotive or aircraft. When factoring the power-to-weight ratio in a wheel-driven vehicle, the numbers can be misleading. In this scenario, it must be clearly listed as to whether the factoring is completed using flywheel horsepower or drive-wheel horsepower since the difference can be great.
When recording the horsepower of an engine, it is attached to an engine dynomometer and run at wide-open throttle for a measured period of time. The dynomometer reads the horsepower produced at the engine's flywheel. This is an accurate indicator of the amount of horsepower that the engine produces while not factoring in any static or parasitic losses. To gain a better understanding of the actual power-to-weight ratio of a vehicle, it should be placed on a chassis dynomometer. By placing the drive wheel or wheels on a chassis dynomometer and taking a reading of the power produced after passing through the transmission and the drive axle, an accurate real world power-to-weight ratio can be arrived at.
In high-performance or racing applications, discovering the power-to-weight ratio of a vehicle can be a very useful bit of information when tuning the engine or suspension. Tire size, shock absorber selection and even braking systems can be better understood and engineered when the power-to-weight ratio is accurately understood. In endurance races, fuel mileage and consumption can be factored using this power and weight information. In a production vehicle such as a pick-up truck, engine size and payload can be factored by using the power and weight information to develop a drive line package. Towing capacities and ratings also benefit from the understanding of the power available as compared to the curb weight of the vehicle.
In commercial vehicles such as tractor-trailer semi rigs, both the empty as well as the loaded weight must be factored into the power-to-weight ratio equation. Aircraft are also subject to the empty and loaded equations in order to properly determine fuel needs for carrying a payload of cargo or passengers. In military applications, the capacity to carry bombs is directly linked to the power available for the intended weight of the payload.