Velocity ratio, occasionally referred to as distance ratio, is a comparison of the amount of force an object, such as a car, is creating in comparison with the other forces around it which act against it. When a car moves, the amount of force applied to the car's wheels by the engine is stronger than forces such as gravity which hold the car in place, and thus the car moves forward. The engine must supply enough power to overcome the vehicle's own weight, friction from the surface of the road, and the force of gravity pushing down on the car. To calculate velocity ratio, divide the force working against an object by the force the object itself is exerting. If it were possible to drive the car in an environment where no forces inhibited its movement, this movement would be known as its mechanical advantage instead of velocity ratio because the driver could calculate exactly how much power would be needed to move the car at a certain speed over a certain distance.
Part of the formula for measuring velocity comes from Isaac Newton's second law of motion which states that F=ma. This means multiplying an object’s acceleration by its mass gives its force. Velocity describes an object's speed and the direction it is traveling. A car's velocity might be 70 mph (112.7 kph) heading north. The second part of the velocity ratio simply compares this force to the forces acting against it to determine if any work is done.
In physics, for an object to do work, it must put forth an effort, or force, that results in movement. If the car doesn't have enough force to move, it is creating effort but getting no work done and, as a result, is not going anywhere. When a driver wants to speed up in her car, she is putting another force known as acceleration into motion. When she presses the gas pedal, the car engine works harder to move the car forward faster. This changes the velocity ratio by requiring the car to exert more force to overcome the forces that keep it moving at a slower speed.
Aside from increasing an engine's power, car owners can get a more speed-efficient vehicle by reducing the forces acting on the car. Smoother roads with less friction, tires that provide better surface traction, and a lighter car all contribute to reducing the velocity ratio and simultaneously improving the car's performance and efficiency. More efficient cars get more work done with less power than less efficient cars.