Performance optimization allows the FMS to determine the best or most economical speed to fly in level flight. This is often called the ECON speed. This is based on the cost index, which is entered to give a weighting between speed and fuel efficiency. The cost index is calculated by dividing the per-hour cost of operating the plane by the cost of fuel. Generally a cost index of 999 gives ECON speeds as fast as possible without consideration of fuel and a cost index of zero gives maximum efficiency. ECON mode is the VNAV speed used by most airliners in cruise.