Throttle response or vehicle responsiveness is a measure of how quickly a vehicle's prime mover, such as an internal combustion engine, can increase its power output in response to a driver's request for acceleration. Throttles are not used in diesel engines, but the term throttle can be used to refer to any input that modulates the power output of a vehicle's prime mover. Increased throttle response is often confused with increased power (Since increasing throttle response reduces the time needed to reach higher RPM speeds and consequently provides immediate access to an internal combustion engine's power and makes a slow car equipped with that engine, for example, feel quicker[1]) but is more accurately described as time rate of change of power levels.[2]