For jet aircraft, "long-range cruise" speed (LRC) is typically chosen to give 1% less fuel efficiency than maximum range speed, because this results in a 3-5% increase in speed. However, fuel is not the only marginal cost in airline operations, so the speed for most economical operation (ECON) is chosen based on the cost index (CI), which is the ratio of time cost to fuel cost.[2] Cost index can be given in "Boeing" or "English" units as ($/hr)/(cents/lb), equivalent to 100 lb/hr.[3][4] A typical cost index in these units might be anywhere from 5 to 150.[5] Alternatively cost index can be given in metric or "Airbus" units of kg/min.[3][4] In the presence of a tailwind, ECON speed will be reduced to take advantage of the tailwind, whereas in a headwind, ECON speed will be increased to avoid the penalty of the headwind. In a strong tailwind, the aircraft can fly at a low speed to conserve fuel, while the tailwind does most of the work getting the aircraft to its destination.